FOUNDATION – 2021

Klinefelter’s syndrome (47,XXY) is a genetic disorder caused by the presence of an extra X chromosome in males. It occurs due to nondisjunction during meiosis, which leads to an abnormal chromosome number.

• Advanced maternal age (women over 35) increases the risk of nondisjunction, making it a risk factor for Klinefelter’s syndrome.
• Affected males typically exhibit tall stature, small testes, gynecomastia (breast development), reduced fertility, and learning difficulties.

This syndrome is a type of aneuploidy, meaning an abnormal number of chromosomes.

Why the Other Options Are Wrong:

1. Low Maternal Age ❌

   • There is no known association between young maternal age and Klinefelter’s syndrome.
   • The risk is higher with increasing maternal age due to aging eggs and increased chances of nondisjunction.

2. Radiation ❌

   • While radiation can cause DNA damage and mutations, it is not a known cause of Klinefelter’s syndrome, which results from chromosomal nondisjunction.

3. Drug Reaction ❌

   • Drug exposure during pregnancy does not cause Klinefelter’s syndrome because it is a chromosomal disorder, not a teratogenic effect.

4. None of These ❌

   • Incorrect! High maternal age is a well-established risk factor.

The esophagus is lined by non-keratinized stratified squamous epithelium, which is specialized for protection against mechanical stress from food passage. This type of epithelium is resistant to friction but lacks keratinization since it remains moist.

• Location of non-keratinized stratified squamous epithelium:
  • Esophagus
  • Oral cavity
  • Pharynx
  • Vagina
  • Anal canal (lower part)

Why the Other Options Are Wrong:

1. Sigmoid Colon → ❌ Wrong
   • The colon, including the sigmoid colon, is lined by simple columnar epithelium with goblet cells, which secrete mucus for lubrication.
2. Rectum → ❌ Partially Correct
   • The rectum is mostly lined by simple columnar epithelium.
   • However, the lower part of the anal canal transitions to non-keratinized and keratinized stratified squamous epithelium.
3. Stomach → ❌ Wrong
   • The stomach is lined by simple columnar epithelium, which contains mucus-secreting cells for protection against gastric acid.
4. Colon → ❌ Wrong
   • Like the sigmoid colon, the entire colon is lined by simple columnar epithelium with goblet cells, not stratified squamous epithelium.

Myoepithelial cells are specialized contractile cells found in glandular tissues, such as salivary glands, sweat glands, and mammary glands. These cells contain contractile filaments (actin and myosin), similar to smooth muscle cells, allowing them to contract and help in the secretion of glandular products.

Their contraction is regulated by the autonomic nervous system and hormonal signals (e.g., oxytocin in the mammary glands for milk ejection).

Why the Other Options Are Incorrect:

1. Epithelial cells

   • ❌ Incorrect because regular epithelial cells serve as protective or absorptive barriers and do not contain contractile filaments.

2. Serous cells

   • ❌ Incorrect because serous cells are glandular cells that secrete watery fluids rich in proteins (e.g., digestive enzymes in the salivary glands) but do not have contractile properties.

3. Blood cells

   • ❌ Incorrect because blood cells (RBCs, WBCs, platelets) do not have contractile filaments; their primary function is transport and immune response.

4. Mucous cells

   • ❌ Incorrect because mucous cells secrete mucins, which form mucus, but they do not contract.

In female oogenesis, oocytes are arrested in the diplotene stage of prophase I from fetal life until puberty.

• During fetal development, primary oocytes begin meiosis but are halted at the diplotene stage of prophase I due to the influence of oocyte maturation inhibitor (OMI) from surrounding granulosa cells.
• This arrest persists until puberty, when a few oocytes resume meiosis in response to luteinizing hormone (LH) during each menstrual cycle.
• The secondary arrest occurs at metaphase II and is only completed if fertilization occurs.

Why the Other Options Are Wrong:

1. Zygotene ❌

   • This is the stage where homologous chromosomes begin to pair (synapsis).
   • The oocyte is not arrested at this stage but continues progressing through meiosis.

2. Leptotene ❌

   • This is the earliest stage of prophase I, where chromosomes begin to condense but do not yet interact significantly.
   • Arrest occurs later, at diplotene.

3. Diakinesis ❌

   • This is the final stage of prophase I, where chromosomes are fully condensed and ready for metaphase I.
   • Arrest occurs earlier, at diplotene, before diakinesis.

4. Pachytene ❌

   • This stage is when crossing over (genetic recombination) occurs.
   • Oocytes do not arrest at this stage; they continue until diplotene, where arrest occurs.

Tight junctions (Zonula occludens) are specialized cell-to-cell junctions that create a barrier to prevent the movement of extracellular fluid and molecules between adjacent epithelial cells. They are crucial in maintaining the integrity of selective permeability, particularly in tissues such as the intestinal epithelium and blood-brain barrier.

These junctions involve transmembrane proteins like occludin and claudin, which help seal the space between cells and prevent the paracellular transport of substances.

Why the Other Options Are Incorrect:

1. Desmosome (Macula adherens)

   • Incorrect because desmosomes provide mechanical strength, linking cells together via intermediate filaments but do not prevent extracellular fluid movement.

2. Macula adherens

   • This is another name for desmosomes, which are primarily involved in cell adhesion, not in forming a tight seal.

3. Gap junction

   • Incorrect because gap junctions allow direct communication between adjacent cells by permitting the exchange of ions and small molecules. They do not prevent extracellular fluid movement.

4. Zonula adherens (Adherens junctions)

   • Incorrect because adherens junctions primarily function in cell adhesion by linking actin filaments of adjacent cells. They are important in maintaining tissue integrity but do not form a fluid-tight seal like tight junctions.

Competitive inhibition occurs when a substance (inhibitor) binds directly to the active site of an enzyme, competing with the substrate for binding. Since both the substrate and inhibitor vie for the same site, the inhibitor prevents the enzyme from catalyzing the reaction unless more substrate is added to outcompete the inhibitor.

This type of inhibition increases the apparent Km (Michaelis constant) but does not change the Vmax because adding more substrate can eventually overcome the inhibition.

Why the Other Options Are Wrong:

1. Allosteric Inhibition

   • In this type of inhibition, the inhibitor binds at a site other than the active site (called the allosteric site).
   • This binding changes the enzyme’s shape, making it less effective or completely inactive.
   • Since the inhibitor does not compete for the active site, this is not competitive inhibition.

2. Uncompetitive Inhibition

   • Here, the inhibitor binds only to the enzyme-substrate complex, not the free enzyme.
   • This type of inhibition lowers both Km and Vmax and cannot be reversed by increasing substrate concentration.
   • Since the inhibitor does not compete with the substrate for the active site, this is not competitive inhibition.

3. Irreversible Inhibition

   • In this type, the inhibitor binds permanently to the enzyme, often by forming covalent bonds.
   • This permanently deactivates the enzyme and cannot be overcome by adding more substrate.
   • Competitive inhibition, on the other hand, is reversible.

4. Non-Competitive Inhibition

   • Here, the inhibitor binds away from the active site and alters the enzyme’s shape.
   • It reduces the enzyme’s activity without being affected by substrate concentration.
   • Unlike competitive inhibition, increasing substrate concentration does not overcome non-competitive inhibition.

Menkes disease is an X-linked recessive disorder caused by mutations in the ATP7A gene, which encodes a copper-transporting ATPase. This leads to impaired copper absorption and transport, resulting in systemic copper deficiency.

Copper is crucial for the function of lysyl oxidase, an enzyme responsible for cross-linking collagen and elastin fibers. The defective enzyme leads to weakened connective tissue, causing:

• Brittle, kinky hair (pili torti)
• Failure to thrive, developmental delay
• Hypotonia (weak muscle tone)
• Arterial fragility leading to aneurysms

Menkes disease is fatal in early childhood if untreated.

Why the Other Options Are Wrong:

1. Manganese –
   • Required for the function of glycosyltransferases and superoxide dismutase.
   • Incorrect because its deficiency does not cause defective collagen cross-linking.
2. Magnesium –
   • Essential for enzymatic reactions, nerve function, and muscle contraction.
   • Incorrect because its deficiency does not cause Menkes disease.
3. Zinc –
   • Important for collagenase activity, wound healing, and immune function.
   • Incorrect because zinc is not involved in lysyl oxidase function.
4. Calcium –
   • Needed for bone mineralization, muscle contraction, and neurotransmission.
   • Incorrect because it is not involved in collagen or elastin cross-linking.

The thoracic duct is the largest lymphatic vessel in the body and is responsible for draining three-fourths of the body’s lymph. It collects lymph from:

• Both lower limbs
• Abdomen
• Left half of the thorax
• Left upper limb
• Left half of the head and neck

The right half of the head and neck is drained by the right lymphatic duct, which also drains:

• Right upper limb
• Right thorax
• Right lung and right side of the heart

Thus, the right half of the head and neck is NOT drained by the thoracic duct, making it the correct answer.

Why the Other Options Are Wrong:

1. Left half of head and neck –
   • The thoracic duct drains the left side of the head and neck.
   • Incorrect because this region is indeed drained by the thoracic duct.
2. Right lower limb –
   • The thoracic duct collects lymph from both lower limbs via the cisterna chyli.
   • Incorrect because the right lower limb is drained by the thoracic duct.
3. Left lower limb –
   • The left lower limb drains into the cisterna chyli, which continues as the thoracic duct.
   • Incorrect because this region is drained by the thoracic duct.
4. All of these –
   • Since the right half of the head and neck is NOT drained by the thoracic duct, this answer is incorrect.

Stratified squamous epithelium (keratinized) is specialized for protection, particularly in areas exposed to mechanical stress, friction, and desiccation. This epithelium consists of multiple layers of cells, with the outermost layers composed of dead, keratinized cells that form a tough, water-resistant barrier. It is found in regions that require maximum protection, such as:

• Skin (epidermis)
• Palate and gingiva (oral cavity)
• Vocal cords

Keratinization provides resistance to abrasion, dehydration, and pathogen entry.

Why the Other Options Are Wrong:

1. Pseudostratified Columnar Epithelium → ❌ Wrong
   • This type is primarily found in the respiratory tract (e.g., trachea, bronchi) and is involved in secretion and movement of mucus via cilia, rather than protection.
2. Simple Cuboidal Epithelium → ❌ Wrong
   • Found in glandular ducts, kidney tubules, and thyroid follicles, this epithelium is mainly involved in secretion and absorption, not protection.
3. Transitional Epithelium → ❌ Wrong
   • Found in the urinary bladder and ureters, it allows for stretching and expansion, rather than providing primary protection.
4. Stratified Squamous Epithelium (Non-Keratinized) → ❌ Wrong
   • While this epithelium provides some protection, it lacks keratin, making it less resistant to mechanical stress than the keratinized version. It is found in moist surfaces like the oral cavity, esophagus, and vagina.

The extracellular fluid (ECF) contains high sodium (Na⁺) and low potassium (K⁺) concentrations, while the intracellular fluid (ICF) contains the opposite (high K⁺, low Na⁺). This balance is crucial for nerve signaling, muscle contraction, and maintaining resting membrane potential.

Normal ECF Concentrations:

• Sodium (Na⁺): ~135-145 mEq/L
  • Most abundant extracellular cation.
  • Plays a key role in osmotic balance, nerve function, and blood pressure regulation.
• Potassium (K⁺): ~3.5-5.0 mEq/L
  • Major intracellular cation (higher inside cells, low outside).
  • Critical for maintaining resting membrane potential and action potentials.

The given option Na⁺ = 142 mEq/L and K⁺ = 4.2 mEq/L falls within the normal physiological range, making it the correct answer.

Why the Other Options Are Wrong:

1. Na⁺ = 145 mEq/L and K⁺ = 4.9 mEq/L – (Sodium on the higher end, potassium slightly elevated but still normal)

   • While 145 mEq/L Na⁺ is at the upper limit of normal, and 4.9 mEq/L K⁺ is also slightly high, the best choice for “normal values” is 142/4.2.

2. Na⁺ = 138 mEq/L and K⁺ = 4.6 mEq/L – (Sodium on the lower end, potassium slightly above normal)

   • 138 mEq/L Na⁺ is on the lower end of normal, and 4.6 mEq/L K⁺ is slightly above the preferred normal value.

3. Na⁺ = 130 mEq/L and K⁺ = 5.6 mEq/L – (Indicates Hyponatremia and Hyperkalemia, Abnormal Values)

   • 130 mEq/L Na⁺ suggests hyponatremia, which can cause confusion, seizures, and coma.
   • 5.6 mEq/L K⁺ indicates hyperkalemia, which can cause cardiac arrhythmias.

4. Na⁺ = 140 mEq/L and K⁺ = 4.0 mEq/L – (Very Close to Normal, But Slightly Less Accurate than 142/4.2 mEq/L)

   • This is almost correct, but 142/4.2 is more precise based on standard physiological values.

The ureter is lined by transitional epithelium, which is highly specialized to accommodate the fluctuating volume of urine passing through.

• Transitional epithelium is stratified and consists of cells that can change shape (from cuboidal to squamous) depending on the degree of stretch.
• It is impermeable to urine, preventing leakage of toxic substances into underlying tissues.
• This type of epithelium is exclusive to the urinary system and is found in the renal pelvis, ureters, bladder, and proximal urethra.

Why the Other Options Are Wrong:

1. Pseudostratified epithelium –
   • Found in the respiratory tract (trachea, bronchi) and male reproductive system (epididymis, vas deferens).
   • Incorrect because the ureter does not have ciliated pseudostratified epithelium.
2. Stratified columnar epithelium –
   • Found in parts of the conjunctiva of the eye, male urethra, and large ducts of salivary glands.
   • Incorrect because the ureter does not have columnar cells in its lining.
3. Simple columnar epithelium –
   • Found in the stomach, intestines, gallbladder, and uterine tubes.
   • Incorrect because the ureter does not have a single layer of columnar cells.
4. Simple cuboidal epithelium –
   • Found in the kidney tubules and small ducts of glands.
   • Incorrect because the ureter requires a more protective, stretchable epithelium.

MicroRNA (miRNA) plays a crucial role in post-transcriptional gene regulation by binding to complementary sequences on target mRNA molecules, leading to mRNA degradation or translation inhibition. miRNAs are small, non-coding RNA molecules that fine-tune gene expression and are involved in cell differentiation, apoptosis, and development.

Why the Other Options Are Wrong:

1. ssRNA (Single-Stranded RNA) → ❌ Wrong
   • While some viruses use ssRNA as their genetic material, ssRNA itself is not a specific regulator of gene expression in eukaryotic cells. It is a broad category rather than a direct regulator.
2. mRNA (Messenger RNA) → ❌ Wrong
   • mRNA carries genetic information from DNA to the ribosome for protein synthesis, but it does not regulate gene expression by itself. However, its stability and translation can be regulated by miRNA.
3. rRNA (Ribosomal RNA) → ❌ Wrong
   • rRNA is a structural and enzymatic component of ribosomes and is essential for protein synthesis, but it does not play a direct role in gene regulation.
4. tRNA (Transfer RNA) → ❌ Wrong
   • tRNA is involved in protein synthesis, as it helps in decoding mRNA by bringing amino acids to the ribosome. It does not regulate gene expression.

Ectopic pregnancy occurs when a fertilized ovum implants outside the uterine cavity. The most common site for this abnormal implantation is the ampulla of the uterine tube, which accounts for 70–80% of ectopic pregnancies.

• The ampulla is the widest and most common site of fertilization. If there is a delay in the movement of the fertilized ovum (due to tubal damage, infection, or scarring), it can implant here instead of reaching the uterus.
• Rupture of an ectopic pregnancy in the ampulla can cause life-threatening hemorrhage and requires emergency management.

Why the Other Options Are Wrong:

1. Intramural uterine tube –
   • This refers to the portion of the fallopian tube embedded within the uterine wall. While an ectopic pregnancy can occur here, it is less common.
   • Incorrect because it accounts for only 2–4% of ectopic pregnancies.
2. Isthmus of the fallopian tube –
   • The isthmus is a narrow section of the fallopian tube closer to the uterus. While it is a possible site, it is less common than the ampulla.
   • Incorrect because it accounts for 12% of ectopic pregnancies.
3. Rectouterine pouch (Pouch of Douglas) –
   • This is the lowest part of the peritoneal cavity, located between the uterus and rectum.
   • Incorrect because an ectopic pregnancy does not commonly implant here; rather, ruptured ectopic pregnancies can cause fluid or blood to accumulate here.
4. Abdominal cavity –
   • Abdominal pregnancies are rare (~1%) and occur when the fertilized egg implants on the peritoneal surfaces (e.g., bowel, omentum).
   • Incorrect because abdominal ectopic pregnancies are much less frequent than tubal ectopic pregnancies.

The primitive streak formation is the first morphological sign of gastrulation, which is the process that converts the bilaminar embryonic disc (epiblast + hypoblast) into a trilaminar disc (ectoderm, mesoderm, and endoderm).

• Primitive streak forms on the dorsal surface of the epiblast.
• It serves as the migration pathway for mesoderm and endoderm formation.
• If remnants of the primitive streak persist after birth, they can develop into a sacrococcygeal teratoma, a tumor containing tissues from all three germ layers.

Why the Other Options Are Wrong:

1. Cleavage → ❌ Wrong
   • Cleavage refers to rapid mitotic divisions of the zygote after fertilization, forming a morula, but no primitive streak forms during cleavage.
2. Implantation → ❌ Wrong
   • Implantation occurs when the blastocyst embeds into the uterine wall, around days 6–7 post-fertilization.
   • The primitive streak appears later, during week 3, after implantation.
3. Neurulation → ❌ Wrong
   • Neurulation involves the formation of the neural tube from the ectoderm, which occurs after gastrulation, not before.
   • The primitive streak disappears before neurulation begins.
4. Blastulation → ❌ Wrong
   • Blastulation is the process of blastocyst formation, occurring before gastrulation. The primitive streak forms only after the blastocyst develops into a bilaminar disc.

The hip joint is a ball and socket joint, which is a type of synovial joint. It consists of:

• Ball: The head of the femur
• Socket: The acetabulum of the pelvis

This joint allows for multiaxial movement, including flexion, extension, abduction, adduction, rotation, and circumduction, making it highly mobile but also prone to dislocation or fractures in elderly individuals due to osteoporosis.

Why the Other Options Are Wrong:

1. Condylar Joint:
   • Condylar (ellipsoid) joints allow movement in two planes (biaxial), such as the wrist joint (radiocarpal joint). The hip joint, however, allows movement in multiple planes (triaxial), making it a ball and socket joint.
2. Hinge Joint:
   • Hinge joints permit movement in one plane (uniaxial), like a door hinge. Examples include the knee joint and elbow joint. The hip joint, however, has more freedom of movement.
3. Pivot Joint:
   • Pivot joints allow rotational movement around a single axis, like the atlantoaxial joint (between C1 and C2 vertebrae). The hip joint, in contrast, allows more complex motions.
4. Saddle Joint:
   • Saddle joints are biaxial and allow movement in two planes, like the thumb’s carpometacarpal joint (CMC joint of the thumb). The hip joint has more degrees of freedom and is not a saddle joint.

During processes like DNA replication and transcription, the double helix unwinds, leading to the formation of supercoils (overwinding or underwinding of the DNA strands). Topoisomerase is the enzyme that relieves this torsional stress by cutting one or both strands of the DNA, allowing it to rotate and then re-ligating the cut strands. This activity is crucial for maintaining proper DNA topology and ensuring that cellular processes involving DNA can proceed without hindrance.

Why the Other Options Are Incorrect

• Single strand DNA:
  This term refers to a form of DNA, not an enzyme. It does not have any catalytic activity to relieve supercoiling.

• DNA ligase:
  DNA ligase is responsible for joining DNA fragments by forming phosphodiester bonds, especially during DNA replication and repair. However, it does not relieve supercoils.

• Helicase:
  Helicase unwinds the DNA double helix by breaking the hydrogen bonds between the bases, but it does not directly resolve the torsional strain (supercoiling) that results from unwinding.

• RNA polymerase:
  RNA polymerase synthesizes RNA from a DNA template during transcription. Although its activity can generate supercoils, it is not responsible for releasing them.

The nucleus is the organelle responsible for storing genetic material (DNA) and controlling cell division. It contains:

• Lamins (proteins) in the nuclear membrane, which provide structural support to the nucleus.
• DNA and histone proteins, which together form chromatin (the material that makes up chromosomes).
• Chromatin, which is a complex of DNA and histone proteins that condenses into chromosomes during cell division.
• The nucleus regulates gene expression and controls cell division through the cell cycle.

Why the Other Options Are Incorrect:

1. Endoplasmic Reticulum (ER)

   • ❌ Incorrect because the ER (both rough and smooth) is involved in protein synthesis (RER) and lipid metabolism (SER) but does not contain DNA, histones, or chromatin.

2. Mitochondrion

   • ❌ Incorrect because mitochondria do have their own DNA, but they do not have histones or chromatin. They primarily function in energy production (ATP synthesis) and apoptosis regulation, not in controlling cell division.

3. Golgi Body

   • ❌ Incorrect because the Golgi apparatus is responsible for modifying, packaging, and sorting proteins for secretion or delivery to other organelles. It has no role in DNA storage or cell division.

4. None of these

   • ❌ Incorrect because the nucleus is the correct answer based on the characteristics given in the question.

In prokaryotic transcription, the sigma (σ) factor of RNA polymerase is responsible for recognizing the promoter region of DNA and initiating transcription.

• The sigma factor helps the RNA polymerase holoenzyme bind to specific promoter sequences, typically at the -10 (TATAAT) and -35 (TTGACA) regions upstream of the transcription start site.
• Once transcription begins, the sigma factor dissociates, allowing the core RNA polymerase to continue elongation.

This specificity allows prokaryotic cells to regulate gene expression efficiently by using different sigma factors under different environmental conditions.

Why the Other Options Are Wrong:

1. Alpha Factor ❌

   • The alpha (α) subunit of RNA polymerase is involved in enzyme assembly and promoter recognition, but it does not directly recognize the promoter site like the sigma factor.

2. Enzyme Assembly Factor ❌

   • No such specific factor exists in prokaryotic transcription.
   • While enzyme subunits assist in RNA polymerase assembly, transcription initiation specifically depends on the sigma factor.

3. Beta Factor ❌

   • The beta (β) and beta-prime (β’) subunits of RNA polymerase are involved in catalytic activity and DNA binding, but they do not recognize the promoter site.

4. Template Binding Factor ❌

   • There is no distinct “template binding factor” in prokaryotic transcription.
   • The core RNA polymerase binds to DNA after sigma factor guides it to the promoter.

Marfan syndrome is a connective tissue disorder caused by a mutation in the FBN1 gene, which encodes the protein fibrillin-1. Fibrillin-1 is crucial for the formation and maintenance of elastic fibers found in various tissues such as the aorta, ligaments, and the zonules of the eye. A defect in this protein results in the weakening of connective tissue, leading to the characteristic features of Marfan syndrome such as lens dislocation (which can lead to vision loss), cardiovascular complications, and skeletal abnormalities.

Why the Other Options Are Incorrect

• Copper: This is an essential trace element involved in various enzymatic processes. Disorders related to copper include Wilson’s disease or Menkes disease, but they have no connection to Marfan syndrome.

• MF 1: This option does not correspond to any recognized gene related to Marfan syndrome. The correct gene is FBN1.

• Trisomy X: This is a chromosomal condition where females have an extra X chromosome. It is unrelated to Marfan syndrome, which is a specific mutation in a single gene, not a chromosomal aneuploidy.

• Collagen: While collagen is a major component of connective tissue and is implicated in other connective tissue disorders (e.g., some types of Ehlers-Danlos syndrome), Marfan syndrome is specifically due to a defect in fibrillin-1, not collagen.

Secondary active transport occurs when a protein moves molecules against their concentration gradient by utilizing the energy from the movement of another ion down its gradient. This differs from primary active transport, which directly uses ATP.

• The sodium-glucose co-transporter (SGLT) is an example of secondary active transport.
• It transports glucose into the cell against its concentration gradient by coupling it with sodium (Na⁺), which moves down its gradient.
• The energy for this transport does not come directly from ATP, but rather from the sodium electrochemical gradient created by the Na⁺/K⁺ pump (which is a primary active transporter).

Why the Other Options Are Wrong:

1. Calcium pump – (Primary Active Transport, Not Secondary Transport)

   • The Ca²⁺ pump (Calcium ATPase) directly uses ATP to move calcium against its gradient.
   • Since it relies on ATP, it is a primary active transporter, not secondary.

2. Na⁺ pump (Likely Referring to Na⁺/K⁺ ATPase) – (Primary Active Transport, Not Secondary Transport)

   • The sodium-potassium pump (Na⁺/K⁺ ATPase) directly hydrolyzes ATP to transport 3 Na⁺ out and 2 K⁺ in.
   • Since it directly uses ATP, it is a primary active transporter.

3. Sodium-potassium pump – (Primary Active Transport, Not Secondary Transport)

   • The Na⁺/K⁺ ATPase (same as above) is a primary active transporter, as it uses ATP directly.

4. None of these – (Incorrect, Because the Sodium-Glucose Co-Transporter Is Secondary Transport)

   • Since the sodium-glucose co-transporter is an example of secondary active transport, this option is wrong.

Sebaceous glands use the holocrine mode of secretion, where the entire cell disintegrates to release its contents.

• In holocrine secretion, the glandular cells accumulate their secretory product (sebum) until they rupture and release it into the duct.
• This is seen in sebaceous glands of the skin, which produce sebum, an oily substance that lubricates the skin and hair.
• Since the whole cell is destroyed in the process, new cells must continuously replace the lost ones.

Why the Other Options Are Wrong:

1. Apocrine ❌

   • Apocrine secretion occurs when a portion of the cell membrane buds off to release the secretory product.
   • It is seen in mammary glands and some sweat glands (apocrine sweat glands of axilla & groin), but not in sebaceous glands.

2. Paracrine ❌

   • Paracrine signaling is a form of cell communication, not a mode of secretion.
   • It involves localized diffusion of signaling molecules to nearby cells (e.g., growth factors, neurotransmitters).

3. Merocrine ❌

   • Merocrine secretion (also called eccrine secretion) involves exocytosis, where secretory vesicles release their content without damaging the cell.
   • Seen in eccrine sweat glands and salivary glands, but not in sebaceous glands.

4. Eccrine ❌

   • Eccrine glands (a type of sweat gland) use merocrine secretion, where secretion occurs via exocytosis.
   • Sebaceous glands do not use this mode of secretion.

For light microscopy, 10% formalin is the fixative of choice. It works by crosslinking proteins, which preserves tissue structure and prevents autolysis and bacterial degradation. This makes it ideal for maintaining cellular details that are crucial for pathological examination.

• Alcohol: While alcohol can fix tissues by precipitation of proteins, it often leads to tissue shrinkage and may not preserve cellular details as effectively as formalin.
• Glyceraldehyde: This is not commonly used as a fixative; its properties do not offer the level of preservation needed for diagnostic histology.
• Osmium tetroxide: Although excellent for fixing lipids and used in electron microscopy, it is too harsh and is not suitable for routine light microscopy.
• Glycerin: Typically used as a mounting medium rather than a fixative, glycerin does not have the chemical properties necessary for tissue fixation.

Phagocytosis is a form of active transport that requires energy (ATP). It is a type of endocytosis, where a cell engulfs large particles, such as bacteria, dead cells, or debris, by extending its membrane to form a phagosome. This process is seen in immune cells like macrophages and neutrophils. Since phagocytosis involves the movement of materials against the concentration gradient and requires vesicle formation, it is energy-dependent.

Why the Other Options Are Wrong:

1. Facilitated Diffusion → ❌ Wrong
   • Facilitated diffusion is passive and does not require energy. It involves carrier or channel proteins that help move molecules (e.g., glucose, ions) down their concentration gradient.
2. Diffusion → ❌ Wrong
   • Simple diffusion is a passive process where molecules move from high to low concentration without requiring ATP. It applies to small, nonpolar molecules like oxygen and carbon dioxide.
3. Filtration → ❌ Wrong
   • Filtration is a passive process driven by hydrostatic pressure, such as in the glomerulus of the kidney, where blood pressure forces fluid through a membrane.
4. Simple Diffusion → ❌ Wrong
   • Like regular diffusion, simple diffusion does not require ATP and happens directly across the lipid bilayer for small, nonpolar molecules.

The brain does not have conventional lymphatic drainage like other organs. Instead, it has a specialized system called the glymphatic system, which facilitates the clearance of waste and excess fluid via cerebrospinal fluid (CSF) circulation, mainly through perivascular spaces. However, it lacks traditional lymphatic vessels.

Why the Other Options Are Wrong:

1. Inguinal region:
   • The inguinal region contains inguinal lymph nodes, which drain lymph from the lower limbs, external genitalia, and parts of the lower abdominal wall.
2. Intestine:
   • The intestine has extensive lymphatic drainage via Peyer’s patches, mesenteric lymph nodes, and lacteals, which absorb dietary fats and transport lymph through the thoracic duct.
3. Axilla:
   • The axilla contains axillary lymph nodes, which are responsible for draining lymph from the upper limb, chest wall, and breast.
4. Breast:
   • The breast has a rich lymphatic drainage system, primarily into the axillary lymph nodes, which play a crucial role in the metastasis of breast cancer.

The carbonic acid-bicarbonate system is quantitatively the most significant buffer system in the blood. It involves the reversible reaction:

CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻

This equilibrium plays a central role in maintaining the blood’s pH. The lungs regulate CO₂ levels through respiration, and the kidneys adjust bicarbonate (HCO₃⁻) reabsorption, making this buffer system highly dynamic and capable of handling significant acid-base challenges.

Why the Other Options Are Incorrect

• Phosphate buffer system:
  Although important in intracellular fluids and the renal tubules, its concentration in blood is much lower than that of the bicarbonate system, making its buffering capacity less significant quantitatively.

• Protein buffer system:
  Proteins, such as albumin, do contribute to buffering by binding hydrogen ions. However, their overall capacity in the blood is less than that of the bicarbonate system, which is the primary regulator of blood pH.

• Hemoglobin:
  Hemoglobin plays a crucial buffering role within red blood cells by binding hydrogen ions. Despite its importance in intracellular buffering, it does not account for the major extracellular buffering capacity compared to the carbonic acid-bicarbonate system.

• Lactic acid:
  Lactic acid is a metabolic byproduct that can contribute to acidification during anaerobic metabolism. It is not a buffer system and does not significantly contribute to maintaining pH balance.

Bacterial conjugation is a process in which genetic material is transferred from one bacterium to another through direct cell-to-cell contact. This is primarily mediated by a sex pilus (a structure produced by donor bacteria with an F (fertility) plasmid).

• The donor bacterium transfers a copy of its plasmid DNA to the recipient.
• This allows for the spread of antibiotic resistance genes and other genetic traits, making it an important mechanism in bacterial evolution.

Why the Other Options Are Wrong:

1. Lysis of bacteria – (Occurs in transduction, not conjugation)

   • Lysis refers to the breaking open of a bacterial cell, which happens in bacteriophage infection (as seen in transduction), but not in conjugation.

2. None of these – (Incorrect, as conjugation involves genetic exchange)

   • Since bacterial conjugation clearly involves genetic exchange, this option is incorrect.

3. Growth of bacteria – (Bacteria do not grow due to conjugation itself)

   • Conjugation does not directly cause bacterial growth; it only transfers genetic material.
   • Bacterial growth occurs through binary fission, which is a separate process.

4. Cells joining together to form a bigger cell – (Bacteria do not fuse into larger cells)

   • Unlike some eukaryotic cells, bacteria do not fuse to form bigger cells.
   • They remain separate entities even after conjugation.

In skeletal muscle, the T-tubules (transverse tubules) are invaginations of the sarcolemma that help propagate action potentials deep into the muscle fiber. These T-tubules are located at the junction between the A-band and I-band of each sarcomere, ensuring synchronized muscle contraction.

Why the Other Options Are Incorrect:

1. “Each muscle cell is separated by an intercalated disc”

   • ❌ Incorrect because intercalated discs are a feature of cardiac muscle, not skeletal muscle. These discs help in electrical coupling and mechanical connection of cardiac muscle cells.

2. “T-tubules form a diad with sarcoplasmic reticulum”

   • ❌ Incorrect because in skeletal muscle, the T-tubules form a triad (one T-tubule with two terminal cisternae of the sarcoplasmic reticulum).
   • A diad (one T-tubule + one terminal cisterna) is characteristic of cardiac muscle.

3. “Each muscle cell has a single nucleus”

   • ❌ Incorrect because skeletal muscle fibers are multinucleated. This happens due to the fusion of myoblasts during development.

4. “All of these”

   • ❌ Incorrect because multiple statements are false. The only correct statement is that T-tubules are in between the A-band and I-band.

Zonula occludens (tight junctions) are specialized cell junctions where the outer leaflets of the plasma membranes of adjacent cells fuse together, creating a seal that prevents the passage of molecules between cells.

• They are composed of occludin and claudin proteins.
• These junctions are crucial in epithelial and endothelial barriers, such as the intestinal lining and blood-brain barrier, to regulate the movement of substances between cells (paracellular transport).

Why the Other Options Are Incorrect:

1. Macula Adherens (Desmosomes)

   • ❌ Incorrect because desmosomes do not fuse membranes; instead, they provide mechanical strength by linking adjacent cells through intermediate filaments.

2. Gap Junctions

   • ❌ Incorrect because gap junctions create communication channels, allowing small molecules and ions to pass between cells. They do not involve membrane fusion.

3. Zonula Adherens (Adherens Junctions)

   • ❌ Incorrect because adherens junctions connect adjacent cells using actin filaments, but they do not fuse the membranes.

4. None of these

   • ❌ Incorrect because Zonula Occludens (tight junctions) is the correct answer.

• The M line is a structural protein line found at the center of the sarcomere (the functional unit of striated muscles).
• It serves as the attachment site for thick filaments (myosin) and helps stabilize their position.
• The sarcomere is organized as follows:
  • Z line: Defines the boundary of the sarcomere.
  • I band: Contains only thin filaments (actin).
  • A band: Overlapping region of thick and thin filaments.
  • H zone: Central region of the A band with only thick filaments.
  • M line: The middle of the H zone, anchoring thick filaments.

Why the Other Options Are Incorrect:

1. “Voluntary muscles are non-striated” (Incorrect)

   • Skeletal muscle (which is voluntary) is striated, meaning it has visible alternating dark and light bands under a microscope.
   • Non-striated muscles refer to smooth muscles, which are involuntary, not voluntary.

2. “Skeletal muscles have multiple nuclei located centrally” (Incorrect)

   • Skeletal muscle cells are multinucleated, but their nuclei are located at the periphery, not centrally.
   • This arrangement allows the center of the cell to be densely packed with contractile proteins.

3. “Cardiac muscles have several nuclei at the periphery” (Incorrect)

   • Cardiac muscle fibers typically have one or two centrally located nuclei.
   • They also have intercalated discs for electrical and mechanical connectivity between cells.

4. “Cross-section of smooth muscles shows cells with single nuclei located peripherally” (Incorrect)

   • Smooth muscle cells are uninucleated, but the nucleus is centrally located, not at the periphery.
   • Smooth muscle appears spindle-shaped in longitudinal section and round with a single central nucleus in cross-section.

Transferases are enzymes that catalyze the transfer of functional groups (such as methyl, amino, or phosphate groups) from one molecule (donor) to another (acceptor). They play a crucial role in metabolic pathways, including amino acid metabolism, glycolysis, and nucleotide biosynthesis.

Examples of transferases:

• Kinases → Transfer phosphate groups (e.g., Hexokinase in glycolysis)
• Aminotransferases (Transaminases) → Transfer amino groups (e.g., Alanine transaminase [ALT] in amino acid metabolism)
• Methyltransferases → Transfer methyl groups (e.g., DNA methyltransferase in gene regulation)

Why the Other Options Are Wrong:

1. Lyases → ❌ Wrong
   • Lyases catalyze the breaking of bonds (C-C, C-N, C-O, etc.) without using water or redox reactions.
   • Example: Aldolase in glycolysis.
2. Hydroxylases → ❌ Wrong
   • Hydroxylases are a subclass of oxidoreductases that add hydroxyl (-OH) groups to substrates, often using oxygen.
   • Example: Tyrosine hydroxylase in dopamine synthesis.
3. Hydrolases → ❌ Wrong
   • Hydrolases catalyze the breaking of bonds using water (hydrolysis).
   • Example: Lipase (breaks down fats) and Amylase (breaks down starch).
4. Oxidases → ❌ Wrong
   • Oxidases belong to the oxidoreductase family and catalyze oxidation-reduction (redox) reactions, often using oxygen.
   • Example: Cytochrome oxidase in the electron transport chain (ETC).

Okazaki fragments are short segments of newly synthesized DNA that are formed on the lagging strand during DNA replication.

How DNA Replication Works:

• DNA polymerase can only synthesize DNA in the 5′ → 3′ direction.
• Since the two strands of DNA are antiparallel, one strand is synthesized continuously (leading strand), while the other is synthesized discontinuously (lagging strand).
• The lagging strand runs in the 5′ → 3′ direction relative to the replication fork, meaning DNA polymerase must synthesize in the opposite direction (away from the fork).
• To accommodate this, short Okazaki fragments are synthesized in the 5′ → 3′ direction, later joined by DNA ligase.

Why the Other Options Are Wrong:

1. Leading, 5′-3′ – (Incorrect, Because the Leading Strand Is Synthesized Continuously, Not in Fragments)

   • The leading strand is synthesized continuously in the 5′ → 3′ direction toward the replication fork.
   • Okazaki fragments are only formed on the lagging strand.

2. Leading, 3′-5′ – (Incorrect, Because DNA Is Always Synthesized in the 5′ → 3′ Direction)

   • DNA polymerase cannot synthesize in the 3′ → 5′ direction.
   • The leading strand is synthesized 5′ → 3′, so this option is incorrect.

3. Lagging, 3′-5′ – (Incorrect, Because Okazaki Fragments Are Synthesized in the 5′ → 3′ Direction, Not 3′ → 5′)

   • The template strand runs 3′ → 5′, but Okazaki fragments are synthesized in the 5′ → 3′ direction.

4. Both same – (Incorrect, Because Only the Lagging Strand Has Okazaki Fragments, Not the Leading Strand)

   • Okazaki fragments occur only on the lagging strand, not on both strands.

The notochord forms as a result of the migration of cells from the primitive pit, which is located within the primitive node (a central structure in early embryonic development).

• During gastrulation, epiblast cells move through the primitive streak and primitive pit and migrate cranially to form a midline rod-like structure known as the notochordal process.
• This structure eventually develops into the notochord, which serves as a critical signaling center for neural tube and somite formation.

The notochord:

• Acts as the primary inducer for neural plate formation (leading to the nervous system).
• Serves as the axial skeleton precursor, providing mechanical support to the developing embryo.
• Contributes to vertebral column development, with remnants persisting as the nucleus pulposus of intervertebral discs.

Why the Other Options Are Wrong:

1. Ectoderm – (Forms from Epiblast, Not from Primitive Pit Migration)

   • The ectoderm is derived from epiblast cells that remain on the surface and do not migrate through the primitive pit.
   • It gives rise to structures like the nervous system, skin, and sensory organs.

2. Endoderm – (Forms from Migration Through the Primitive Streak, Not Specifically the Primitive Pit)

   • The endoderm forms from cells that migrate through the primitive streak, not directly from the primitive pit.
   • It gives rise to the gut lining, liver, pancreas, and respiratory tract.

3. Mesoderm – (Forms from the Primitive Streak, Not Directly the Primitive Pit)

   • The mesoderm arises from epiblast cells migrating through the primitive streak, giving rise to structures like muscles, bones, the cardiovascular system, and kidneys.
   • While some notochordal precursors are mesodermal, the notochord itself is distinct and forms from primitive pit cells migrating cranially.

4. Allantois – (Forms as an Outpouching of the Hindgut, Not from the Primitive Pit)

   • The allantois is an outgrowth from the yolk sac and forms part of the umbilical cord and urinary system.
   • It does not arise from the primitive pit.

• Affinity refers to how strongly a drug binds to its receptor.
• It is determined by the chemical interactions between the drug and the receptor, such as hydrogen bonds, ionic bonds, and van der Waals forces.
• A drug with high affinity binds more strongly and effectively to its target receptor, even at low concentrations.
• Affinity is a key factor in determining the potency of a drug but is different from efficacy, which refers to the ability of a drug to produce a biological effect after binding.

Why the Other Options Are Incorrect:

1. “Measure of bioavailability of a drug” (Incorrect)

   • Bioavailability refers to the fraction of an administered drug that reaches the systemic circulation in an active form.
   • It is influenced by absorption, metabolism, and first-pass effect, not receptor binding.

2. “Measure of rate at which it is eliminated” (Incorrect)

   • This describes drug clearance and half-life, which are related to pharmacokinetics, not affinity.
   • Affinity deals with how a drug binds to receptors, not how fast it is eliminated.

3. “Measure of inhibiting potency of a drug” (Incorrect but tricky)

   • Potency refers to the dose required to achieve a given effect and is influenced by both affinity and efficacy.
   • Inhibiting potency is often measured by IC₅₀ (half-maximal inhibitory concentration), which describes how much drug is needed to inhibit a biological process by 50%.
   • However, affinity alone does not determine inhibitory potency, making this option incorrect.

4. “How tightly drug binds to plasma proteins” (Incorrect)

   • Plasma protein binding affects drug distribution, free drug concentration, and duration of action, but not affinity for receptors.
   • Drugs that are highly bound to plasma proteins (e.g., albumin) may have a longer half-life, but this does not describe their receptor affinity.

Red blood cells (RBCs) undergo osmosis when placed in solutions of different tonicity. A 0.1% sodium chloride (NaCl) solution is hypotonic relative to the normal physiological concentration of NaCl in the body (0.9% NaCl, or isotonic solution).

• In a hypotonic solution, there is a lower concentration of solutes outside the RBC than inside.
• Water moves into the RBC by osmosis to balance the solute concentration.
• This causes the RBCs to swell as they take in water.
• If excessive water enters, the RBCs burst (hemolysis) due to the increased internal pressure.

Why the Other Options Are Incorrect:

1. Shrink and burst

   • ❌ Incorrect because RBCs do not shrink in a hypotonic solution; instead, they swell and lyse. Shrinking (crenation) occurs in hypertonic solutions (e.g., >0.9% NaCl).

2. Remain same

   • ❌ Incorrect because water movement occurs, leading to swelling and bursting of RBCs in a hypotonic solution. The RBCs would remain the same only in an isotonic solution (0.9% NaCl).

3. Swell only

   • ❌ Incorrect because while RBCs initially swell, they eventually burst (lyse) due to excessive water intake.

4. Shrink

   • ❌ Incorrect because shrinking (crenation) occurs in hypertonic solutions, where water moves out of the cell. A 0.1% NaCl solution is hypotonic, causing water to enter instead.

Arachidonic acid (AA) is a polyunsaturated fatty acid (PUFA) that serves as the primary precursor of prostaglandins, thromboxanes, and leukotrienes.

• It is a 20-carbon fatty acid with four double bonds (20:4, ω-6).
• It is stored in cell membranes as part of phospholipids and is released by phospholipase A₂ when needed.
• Once released, it is converted by the cyclooxygenase (COX) enzyme pathway into prostaglandins (PGE₂, PGF₂α, PGD₂), thromboxanes, and prostacyclins.
• These molecules play key roles in inflammation, pain, fever, and platelet aggregation.

Why the Other Options Are Incorrect:

1. Eicosapentaenoic acid (EPA, 20:5, ω-3)

   • ❌ Incorrect because EPA is an omega-3 fatty acid that competes with arachidonic acid for conversion by COX and lipoxygenase (LOX) enzymes, leading to the production of anti-inflammatory prostaglandins (PGE₃) and leukotrienes (LTB₅).
   • However, AA is the main precursor of the classic prostaglandins (PGE₂, PGF₂α, etc.).

2. Linolenic acid (α-linolenic acid, ALA, 18:3, ω-3)

   • ❌ Incorrect because linolenic acid is an omega-3 fatty acid and is not directly involved in prostaglandin synthesis.
   • It is a precursor for EPA and docosahexaenoic acid (DHA), which have anti-inflammatory effects.

3. Omega-3 fatty acids

   • ❌ Incorrect because omega-3 fatty acids (such as ALA, EPA, and DHA) generally produce less inflammatory prostaglandins compared to omega-6 fatty acids like arachidonic acid.
   • While EPA can produce some prostaglandins, AA is the primary precursor.

4. Linoleic acid (18:2, ω-6)

   • ❌ Incorrect because linoleic acid is not a direct precursor of prostaglandins. However, it is converted into arachidonic acid, which then serves as the direct precursor.

The neurons responsible for transmitting sensory signals from the anterior surface of the forearm to the central nervous system (CNS) are pseudounipolar neurons. These neurons are found in the dorsal root ganglia (DRG) of spinal nerves and are specialized for sensory transmission.

• Pseudounipolar neurons have a single process that splits into:
  • Peripheral process: Receives sensory input from the skin, muscles, and joints.
  • Central process: Carries sensory signals into the spinal cord and brainstem.
• These neurons conduct touch, pain, temperature, and proprioception.

Why the Other Options Are Wrong:

1. Bipolar → ❌ Wrong
   • Bipolar neurons are specialized sensory neurons found in the retina (vision), olfactory epithelium (smell), and the vestibulocochlear system (hearing and balance).
   • They are not involved in general sensory transmission from the forearm.
2. Multipolar → ❌ Wrong
   • Multipolar neurons have multiple dendrites and one axon and are primarily motor neurons or interneurons in the CNS (e.g., spinal cord and brain).
   • Sensory neurons from the skin are not multipolar.
3. Motor Neuron → ❌ Wrong
   • Motor neurons carry signals from the CNS to muscles, causing movement (efferent signals).
   • Sensory information is afferent and is carried by pseudounipolar neurons, not motor neurons.
4. Interneuron → ❌ Wrong
   • Interneurons are short neurons that connect other neurons within the CNS (e.g., in reflex arcs).
   • They are not primary sensory neurons responsible for transmitting signals from the periphery.

The correct answer is Their pKa is low.

Here’s why:

• Strong acids completely or nearly completely dissociate in water, meaning they donate their protons (H+) very readily. A high Ka (acid dissociation constant) value indicates a strong acid because it reflects a greater extent of dissociation. Conversely, a low pKa value (which is the negative logarithm of Ka) corresponds to a high Ka and therefore a strong acid. The lower the pKa, the stronger the acid.  

Let’s look at why the other options are incorrect:

• Their Ka is low: As explained above, strong acids have a high Ka, not a low one.

• They dissociate partially: Strong acids dissociate completely or nearly so. Weak acids dissociate partially.  

• Their pH is high: Strong acids, when dissolved in water, make the solution acidic, meaning the pH is low, not high.

• Their pKa is high: A high pKa indicates a weak acid, not a strong one.

   

Gap junctions form tube-like channels between adjacent cells, allowing for direct communication and the passage of ions, nutrients, and signaling molecules.

• In cardiomyocytes (heart muscle cells), gap junctions are essential for electrical coupling and synchronized contraction.
• They are formed by connexins, which create connexon channels, allowing ions like Ca²⁺ and Na⁺ to pass freely between cells.
• This enables the rapid spread of action potentials, ensuring coordinated contraction of the heart.

Why the Other Options Are Wrong:

1. Occluding junction (Tight Junction) – (Incorrect, Because It Acts as a Barrier, Not a Channel)

   • Tight junctions create a seal between cells to prevent leakage (e.g., in the intestines and blood-brain barrier).
   • They do not form tube-like channels or allow electrical communication.

2. Desmosome – (Incorrect, Because It Provides Mechanical Strength, Not Communication)

   • Desmosomes anchor adjacent cells together, providing mechanical strength (e.g., in skin and cardiac muscle).
   • They do not form direct channels for communication like gap junctions.

3. Hemidesmosome – (Incorrect, Because It Attaches Cells to the Basement Membrane, Not to Each Other)

   • Hemidesmosomes attach cells to the extracellular matrix (like the basement membrane), not to adjacent cells.
   • They are not involved in intercellular communication.

4. Tight junction – (Incorrect, Because It Seals Cells Rather Than Connecting Them for Communication)

   • Tight junctions seal adjacent cells, preventing fluid leakage (e.g., in epithelial barriers).
   • They do not facilitate ion passage or electrical conduction.

Turner’s syndrome (45,X or 45,XO) is a chromosomal disorder in females caused by the loss of one X chromosome, resulting in a monosomy (only one sex chromosome instead of two).

• It is the only viable monosomy in humans.
• Individuals are phenotypically female but present with short stature, webbed neck, streak ovaries (infertility), and congenital heart defects (e.g., coarctation of the aorta).
• The correct notation is 45,X, but “XO” is commonly used to emphasize the absence of the second sex chromosome.

Why the Other Options Are Wrong:

1. Klinefelter’s → 45,XXY ❌ (Incorrect!)

   • The correct karyotype for Klinefelter’s syndrome is 47,XXY, not 45,XXY.
   • Males with this condition have an extra X chromosome, leading to tall stature, small testes, infertility, and gynecomastia.

2. Down Syndrome → Trisomy 18 ❌ (Incorrect!)

   • Down syndrome is caused by Trisomy 21 (47,XX+21 or 47,XY+21), not Trisomy 18.
   • Trisomy 18 is Edwards syndrome, which presents with severe developmental delays, congenital heart defects, and characteristic clenched hands with overlapping fingers.

3. Klinefelter’s → 45,XO ❌ (Incorrect!)

   • Klinefelter’s syndrome is 47,XXY, not 45,XO.
   • 45,XO is Turner’s syndrome, which affects females, whereas Klinefelter’s affects males.

4. Turner → 47,XXXY ❌ (Incorrect!)

   • 47,XXXY is a variant of Klinefelter’s syndrome, not Turner’s syndrome.
   • Turner’s syndrome is 45,X (or 45,XO), not 47,XXXY.

The active site of an enzyme is the specific region where substrate binding occurs and the catalytic reaction takes place. This site has a unique shape and chemical environment that facilitates the conversion of substrates into products. It is often composed of amino acid residues that stabilize the transition state and lower the activation energy of the reaction.

Why the Other Options Are Wrong:

1. Co-factor:
   • A co-factor is a non-protein component (e.g., metal ions like Zn²⁺, Mg²⁺) required for enzyme activity, but it is not the site where catalysis occurs. Instead, it assists the active site in performing the reaction.
2. Co-enzyme:
   • A coenzyme is an organic molecule (e.g., NAD⁺, FAD, CoA) that helps enzymes in catalysis by transferring chemical groups. However, it is a helper molecule, not the catalytic site itself.
3. Prosthetic group:
   • A prosthetic group is a permanently attached co-factor or coenzyme that assists enzyme function (e.g., heme in hemoglobin). While it plays a crucial role, catalysis does not occur at the prosthetic group itself.
4. Protein part:
   • The protein part (also called the apoenzyme) forms the structure of the enzyme but does not specifically refer to the catalytic site. Enzymes function through their active sites, which are specific regions of the protein.

Dystrophic calcification is a process in which calcium salts are deposited in areas of tissue damage, particularly in necrotic tissue. Despite the deposition of calcium, the serum calcium levels remain normal. This contrasts with metastatic calcification, where elevated blood calcium levels (often due to conditions like hyperparathyroidism) cause calcium deposits in otherwise normal tissues.

Why the Other Options Are Incorrect

• Fractured bones:
  Fractures heal through a process involving bone remodeling and callus formation; they are not an example of dystrophic calcification.

• Normal calcium levels in blood:
  Although dystrophic calcification occurs with normal blood calcium levels, it is not caused by them. The key factor is the tissue damage and necrosis that provide a nidus for calcium deposition.

• Bone formation:
  Bone formation (osteogenesis) is a normal physiological process and is distinct from pathological calcification seen in dystrophic calcification.

• Hyperparathyroidism:
  Hyperparathyroidism leads to elevated serum calcium levels and results in metastatic calcification, not dystrophic calcification.

A symport is a type of secondary active transport where two or more substances move in the same direction across a membrane, driven by an electrochemical gradient.

• The Na-K-2Cl carrier protein is an example of a symport co-transporter. It is found in the thick ascending limb of the loop of Henle in the kidney and plays a crucial role in the reabsorption of sodium, potassium, and chloride.
• It simultaneously transports one sodium (Na⁺), one potassium (K⁺), and two chloride (Cl⁻) ions into the cell in the same direction.

Why the Other Options Are Wrong:

1. Sodium-hydrogen counter transport (Na⁺/H⁺ exchanger) – (Antiport, Not Symport)

   • This transporter exchanges sodium ions (Na⁺ moving in) with hydrogen ions (H⁺ moving out), meaning they move in opposite directions.
   • This is an example of antiport (counter-transport), not symport.

2. Sodium-calcium counter transport (Na⁺/Ca²⁺ exchanger) – (Antiport, Not Symport)

   • This exchanger moves Na⁺ into the cell and Ca²⁺ out of the cell (or vice versa), which is in opposite directions.
   • Since the movement is opposite, this is antiport, not symport.

3. Hydrogen pump (H⁺ ATPase) – (Primary Active Transport, Not Co-transport)

   • The hydrogen pump actively pumps H⁺ ions out of the cell using ATP, making it a primary active transport mechanism, not a co-transporter (symport or antiport).

4. Sodium-potassium pump (Na⁺/K⁺ ATPase) – (Primary Active Transport, Not Co-transport)

   • The Na⁺/K⁺ ATPase pumps 3 Na⁺ out and 2 K⁺ in, using ATP.
   • This is an example of primary active transport, not a symport.

The joint between the epiphysis and diaphysis of a long bone (such as the tibia) is a primary cartilaginous joint, also known as a synchondrosis. This joint is composed of hyaline cartilage and is found in the growth plates (epiphyseal plates) of growing bones. These joints allow growth but do not permit movement and eventually ossify with age.

In this case, the injury led to epiphyseal plate dislocation, which is common in children and adolescents because their bones are still growing.

Why the Other Options Are Wrong:

1. Gomphosis → ❌ Wrong
   • Gomphosis is a fibrous joint found only in the tooth socket (between teeth and alveolar bone).
   • It does not involve long bones like the tibia.
2. Secondary Cartilaginous (Symphysis) → ❌ Wrong
   • Symphyses are slightly movable joints made of fibrocartilage, found in midline structures like the pubic symphysis and intervertebral discs.
   • The epiphyseal plate contains hyaline cartilage, not fibrocartilage.
3. Fibrous Joint → ❌ Wrong
   • Fibrous joints are immovable and include structures like sutures in the skull.
   • The epiphyseal plate is a cartilaginous joint, not a fibrous one.
4. Synovial Joint → ❌ Wrong
   • Synovial joints are highly movable joints (e.g., knee, hip, shoulder) that contain a joint cavity filled with synovial fluid.
   • The epiphyseal plate does not have a synovial cavity and is not a synovial joint.

Water reaches its maximum density at about 4°C due to the unique behavior of its molecules. As water cools from higher temperatures, the molecules slow down and can pack closer together, increasing the density. However, when the temperature drops below 4°C, hydrogen bonds start organizing the molecules into a more open, hexagonal structure. This structure occupies more space, causing the density to decrease. That’s why water at 0°C (and below) is less dense than at 4°C, which is also why ice floats on liquid water.

Why the Other Options Are Incorrect

• -1°C and -4°C: Under normal conditions, water cannot be in a stable liquid state below 0°C without supercooling. Moreover, even if supercooled water were considered, the density decreases below 4°C because of the onset of the open hexagonal molecular arrangement.
• 0°C: At 0°C, water is at the freezing point, where the molecular structure starts to shift towards the open arrangement seen in ice. This leads to a lower density compared to water at 4°C.
• 10°C: At 10°C, the water molecules have more thermal energy and are less tightly packed compared to the optimal arrangement at 4°C, resulting in a lower density.

Resolution (also called resolving power) is the ability of a microscope or optical system to distinguish two closely spaced objects as separate entities. It refers to the shortest distance between two points that can still be seen as distinct objects rather than a single blurred point.

• Resolution is not the same as magnification. A highly magnified image with poor resolution will appear blurry.

• The resolution of a microscope is determined by the wavelength of light used and the numerical aperture (NA) of the objective lens.

• Formula for Resolution (d):

  d=0.61λNAd = \frac{0.61 \lambda}{NA}d=NA0.61λ​

  where λ = wavelength of light, and NA = numerical aperture of the lens.

• Light microscopes have a resolution limit of about 200 nm, whereas electron microscopes have a much higher resolution (~0.1 nm).

Why the Other Options Are Wrong:

1. Factor by Which the Image is Enlarged ❌

   • This defines magnification, not resolution.
   • Magnification refers to how much larger an object appears but does not determine the clarity of detail.

2. Largest Distance Between Two Points That Can Be Visually Distinguished ❌

   • Incorrect! Resolution is about the smallest distance that can still be differentiated, not the largest.

3. Source of Illumination by Which the Specimen is Visualized ❌

   • This describes the light source (e.g., visible light in optical microscopes or electron beams in electron microscopes).
   • Illumination affects imaging but does not define resolution.

4. None of These ❌

   • Incorrect! The correct definition is already provided as the first option.

The hypodermis and the papillary layer of the dermis are both composed of loose connective tissue, also known as areolar connective tissue.

1. Hypodermis (Subcutaneous Tissue):

   • Made up of loose connective tissue and adipose tissue.
   • Functions as insulation, energy storage, and cushioning for the skin.

2. Papillary Layer of the Dermis:

   • The superficial layer of the dermis.
   • Composed of loose connective tissue with collagen and elastic fibers.
   • Contains capillaries, lymphatics, and sensory neurons.

Why the Other Options Are Wrong:

1. Irregular – (Incorrect, Because Irregular Dense Tissue Is Found in the Reticular Layer, Not the Papillary Layer or Hypodermis)

   • Dense irregular connective tissue is found in the reticular layer of the dermis, not in the hypodermis or papillary layer.
   • It provides mechanical strength and resistance to stretching.

2. Dense – (Incorrect, Because the Papillary Layer and Hypodermis Are Composed of Loose Connective Tissue)

   • While dense connective tissue is found in tendons, ligaments, and the reticular layer of the dermis, it is not a major component of the hypodermis or papillary layer.

3. None of these – (Incorrect, Because Loose Connective Tissue Is the Correct Answer)

   • Since both the hypodermis and papillary dermis contain loose connective tissue, this option is wrong.

Cholesterol has a dual role:

• At high temperatures, it reduces fluidity by stabilizing the membrane and limiting the movement of phospholipids.
• At low temperatures, it prevents membrane solidification by disrupting tight packing of phospholipids, thereby increasing fluidity.

Turner syndrome (45, XO) is a monosomic condition, meaning there is a missing chromosome in an otherwise diploid set.

• Normally, humans have 46 chromosomes (23 pairs), including two sex chromosomes (XX in females, XY in males).
• In Turner syndrome, a female has only one X chromosome (45, XO) instead of two.
• This results in monosomy, where an entire chromosome is missing.

Key Features of Turner Syndrome:

• Short stature
• Webbed neck
• Ovarian dysgenesis (leading to infertility)
• Cardiac abnormalities (e.g., coarctation of the aorta)
• Normal intelligence but possible learning difficulties

Why the Other Options Are Wrong:

1. Disomic condition – (Incorrect, Because Turner Syndrome Has One Missing Chromosome)

   • “Disomy” means having the normal two copies of each chromosome.
   • Turner syndrome has only one X chromosome (monosomy), not two, so this option is incorrect.

2. None of these – (Incorrect, Because Turner Syndrome Clearly Falls Under Monosomy)

   • Turner syndrome is a well-known monosomic condition, so this option is wrong.

3. Trisomic condition – (Incorrect, Because Turner Syndrome Involves a Missing, Not Extra, Chromosome)

   • Trisomy means having an extra chromosome (47 instead of 46).
   • Examples of trisomic conditions include:
     • Down syndrome (Trisomy 21)
     • Edward syndrome (Trisomy 18)
     • Patau syndrome (Trisomy 13)
   • Turner syndrome, however, results from monosomy, not trisomy.

4. Haploid condition – (Incorrect, Because Humans Are Diploid, Not Haploid, at Birth)

   • A haploid cell has only one set of chromosomes (n = 23), which occurs in gametes (sperm and egg).
   • Turner syndrome individuals still have 45 chromosomes (though missing one), meaning they are not haploid but aneuploid.

The parenteral route refers to drug administration bypassing the gastrointestinal (GI) tract, typically via injection (IV, IM, SC, etc.). It provides several advantages over the oral route, the most important being greater absorption and bioavailability.

• When drugs are given orally, they undergo first-pass metabolism in the liver, which can reduce their effectiveness.
• Parenteral administration avoids first-pass metabolism, ensuring a higher percentage of the drug reaches systemic circulation.
• It also provides faster onset of action, especially in intravenous (IV) administration.

Why the Other Options Are Wrong:

1. Cannot be used in emergency ❌

   • Incorrect!
   • The parenteral route is preferred in emergencies because it allows for rapid drug action (e.g., IV administration of epinephrine during anaphylaxis).

2. Takes time to reach the site of action ❌

   • Incorrect!
   • The parenteral route, especially IV, provides immediate drug availability in circulation. IM and SC may take a little longer but are still faster than oral absorption.

3. Cannot be given to unconscious patients ❌

   • Incorrect!
   • One of the main advantages of parenteral administration is that it can be given to unconscious or uncooperative patients who cannot swallow pills.

4. Delayed access to systemic circulation ❌

   • Incorrect!
   • IV administration provides instant systemic circulation access, while IM and SC routes are still faster than oral absorption.

Gastrulation is the process in which the bilaminar embryonic disc (epiblast + hypoblast) is converted into a trilaminar disc composed of the ectoderm, mesoderm, and endoderm. The first morphological sign of gastrulation is the appearance of the primitive streak on the dorsal surface of the epiblast.

The primitive streak serves as the site where epiblast cells migrate inward, forming the three germ layers:

• Ectoderm → Forms skin, nervous system
• Mesoderm → Forms muscles, bones, cardiovascular system
• Endoderm → Forms the gastrointestinal and respiratory tracts

Why the Other Options Are Wrong:

1. Development of Neural Plate → ❌ Wrong
   • The neural plate forms later during neurulation, which occurs after gastrulation. It develops from the ectoderm and eventually gives rise to the central nervous system.
2. Development of Notochord → ❌ Wrong
   • The notochord develops from the mesoderm, but this happens after the formation of the primitive streak. The notochord plays a role in signaling for neural tube formation.
3. Development of Prechordal Plate → ❌ Wrong
   • The prechordal plate is a thickened area of mesoderm that contributes to cranial structures, but it forms after the primitive streak.
4. Development of Neural Tube → ❌ Wrong
   • The neural tube forms during neurulation, which occurs after gastrulation. It originates from the neural plate, not the primitive streak.

Tyrosine is classified as a non-essential amino acid because it can be synthesized in the body from phenylalanine, an essential amino acid. If phenylalanine intake is inadequate (as seen in phenylketonuria, PKU), tyrosine becomes conditionally essential.

Tyrosine plays a crucial role in:

• Neurotransmitter synthesis (dopamine, epinephrine, norepinephrine)
• Thyroid hormone synthesis (T3, T4)
• Melanin production (skin pigmentation)

Why the Other Options Are Wrong:

1. Basic → ❌ Wrong
   • Basic amino acids have positively charged side chains at physiological pH, such as arginine, lysine, and histidine.
   • Tyrosine has a hydroxyl (-OH) group and is not basic.
2. Simple → ❌ Wrong
   • Simple amino acids are those with nonpolar or very basic structures (e.g., glycine, alanine).
   • Tyrosine has a hydroxylated aromatic ring, making it more complex.
3. Essential → ❌ Wrong
   • Essential amino acids cannot be synthesized by the body and must be obtained from the diet (e.g., leucine, lysine, valine).
   • Tyrosine can be synthesized from phenylalanine, so it is non-essential.
4. Acidic → ❌ Wrong
   • Acidic amino acids (e.g., aspartic acid, glutamic acid) have negatively charged side chains.
   • Tyrosine does not have a carboxyl (-COO⁻) functional group in its side chain.

The answer is Epimers.

Explanation:

Epimers are a specific type of stereoisomer that differ in the configuration at only one chiral center (a carbon atom bonded to four different groups). Think of it like two slightly different versions of the same molecule, where just one “part” is flipped. A common example is D-glucose and D-galactose, which are epimers at the C-4 carbon.

Why the other options are not the most accurate:

• Enantiomers: These are stereoisomers that are non-superimposable mirror images of each other. They differ at all chiral centers. While epimers are stereoisomers, “enantiomers” is not the most specific term here.

• Cis isomers: This term is usually used to describe molecules with double bonds where substituents are on the same side (cis) or opposite sides (trans). While related to stereochemistry, it’s not the term used for differences around a single chiral carbon in carbohydrates.

• Stereoisomers: This is a broad term that encompasses both epimers and enantiomers (and other types). While epimers are stereoisomers, the question asks for the specific term.

• Optical isomers: This term is often used interchangeably with enantiomers, referring to molecules that rotate plane-polarized light. Again, while epimers are optically active, “optical isomers” is not the most precise term in this context.

The physical and chemical properties of fatty acids are influenced by:

1. Chain length: Longer chains have higher melting points and are less soluble in water.
2. Degree of saturation: Saturated fatty acids (no double bonds) are more rigid and have higher melting points, while unsaturated fatty acids (with double bonds) are more fluid and have lower melting points.

The size of carbon atoms is constant and does not affect fatty acid properties.

Waxes are formed by the esterification of fatty acids with long-chain alcohols. They are hydrophobic, solid at room temperature, and provide protective coatings in biological systems, such as:

• Plants → Waxes prevent water loss from leaves (cuticle layer).
• Animals → Beeswax is used in honeycombs.
• Humans → Earwax (cerumen) protects the ear canal.

Waxes differ from triglycerides because they involve long-chain alcohols instead of glycerol.

Why the Other Options Are Wrong:

1. Fatty alcohols –
   • These are long-chain alcohols derived from fatty acids, but they are not esters.
   • Incorrect because they are precursors, not the esterified product.
2. Cholesterol –
   • A sterol (steroid alcohol) that is a major component of cell membranes and a precursor to steroid hormones.
   • Incorrect because it is not an ester of fatty acids and alcohols.
3. Steroids –
   • Lipid-based molecules with a four-ring structure, including hormones like testosterone and estrogen.
   • Incorrect because steroids do not involve esterification of fatty acids with long-chain alcohols.
4. Triglycerides –
   • These are glycerol esters of three fatty acids, stored as fat in the body.
   • Incorrect because triglycerides use glycerol, not long-chain alcohols.

Fibronectin is a major glycoprotein of the extracellular matrix (ECM) that also exists in a soluble form in plasma. It plays a crucial role in:

• Cell adhesion
• Wound healing
• Cell migration
• Tissue repair
• Connecting ECM components like collagen and integrins

Plasma fibronectin, also called soluble fibronectin, is produced by hepatocytes (liver cells) and circulates in the blood. It assists in coagulation, immune responses, and tissue remodeling.

Why the Other Options Are Wrong:

1. Laminin ❌

   • Incorrect! Laminin is another ECM glycoprotein, but it is primarily found in the basement membrane rather than in plasma.
   • It helps in cell differentiation, adhesion, and migration, but it is not soluble in plasma.

2. Elastin ❌

   • Incorrect! Elastin is a structural protein responsible for the elasticity of tissues like skin, lungs, and blood vessels.
   • It is not a glycoprotein and does not exist in a soluble form in plasma.

3. Collagen ❌

   • Incorrect! Collagen is the most abundant structural protein in the ECM but is not soluble in plasma.
   • It provides strength and structure to tissues like skin, cartilage, and bones.
   • Unlike fibronectin, collagen is insoluble in its mature form.

4. Proteoglycan ❌

   • Incorrect! Proteoglycans are ECM components that help with hydration and shock absorption, especially in cartilage.
   • While they have glycosylated proteins, they are not typically soluble in plasma like fibronectin.

When a first-year medical student feels nervous during their viva exam, their sympathetic nervous system (SNS) is activated as part of the “fight or flight” response. This leads to the release of epinephrine and norepinephrine, causing:

• Increased heart rate (tachycardia) to pump more blood to muscles and the brain.
• Increased respiratory rate to provide more oxygen.
• Pupil dilation (mydriasis) for better vision.
• Decreased digestive activity (reduced gastrointestinal motility).

Why the Other Options Are Wrong:

1. Increased Gastrointestinal Motility → ❌ Wrong
   • The parasympathetic nervous system (PNS) controls digestion.
   • During stress, SNS suppresses digestion, leading to reduced gastrointestinal motility, not an increase.
2. Vasoconstriction → ✅ Partially Correct, but not the Best Answer
   • SNS causes vasoconstriction in non-essential organs (e.g., gut, skin) to redirect blood to the brain, heart, and muscles.
   • However, the increase in heart rate (cardiac output) is a more direct response to stress.
3. Constriction of Pupil → ❌ Wrong
   • The sympathetic response causes pupil dilation (mydriasis) to enhance vision.
   • Pupil constriction (miosis) is a parasympathetic function.
4. Bronchoconstriction → ❌ Wrong
   • SNS activation leads to bronchodilation, allowing more oxygen intake.
   • Bronchoconstriction is controlled by the parasympathetic nervous system.

Cellulose is a structural polysaccharide that forms the primary component of plant cell walls. It is composed of β-1,4 glycosidic linkages, which provide rigidity and strength to plant cells. Humans lack the cellulase enzyme, so cellulose cannot be digested, making it a major component of insoluble dietary fiber.

• Insoluble fiber (like cellulose) helps in maintaining bowel regularity by adding bulk to stool and promoting digestion.
• Found in: Whole grains, vegetables, fruits, and legumes.

Why the Other Options Are Wrong:

1. Glycogen → ❌ Wrong
   • Glycogen is a storage polysaccharide in animals, not plants.
   • It has α-1,4 and α-1,6 glycosidic linkages, making it highly branched.
2. Chitin → ❌ Wrong
   • Chitin is a structural polysaccharide in fungi and exoskeletons of arthropods (e.g., insects, crustaceans).
   • It is made of N-acetylglucosamine (GlcNAc) units and is not a major component of plant cell walls.
3. Starch → ❌ Wrong
   • Starch is the storage polysaccharide in plants, made up of amylose and amylopectin.
   • It is digestible by humans and does not contribute significantly to insoluble dietary fiber.
4. Dextrin → ❌ Wrong
   • Dextrins are short-chain carbohydrates derived from starch breakdown.
   • They are easily digestible and do not form plant cell walls.

The parasympathetic nervous system (PNS) is responsible for the “rest and digest” functions, promoting relaxation, digestion, and energy conservation. One of its effects is bronchoconstriction, which reduces airflow by narrowing the bronchi. This is mediated by the vagus nerve (CN X) through acetylcholine acting on muscarinic receptors (M3 receptors) in the lungs.

Why the Other Options Are Incorrect:

1. Ejaculation

   • ❌ Incorrect because ejaculation is controlled by the sympathetic nervous system (“fight or flight” response).
   • The mnemonic “Point and Shoot” helps remember that:
     • Parasympathetic controls erection (Point)
     • Sympathetic controls ejaculation (Shoot)

2. Dilation of pupil (Mydriasis)

   • ❌ Incorrect because pupil dilation is a sympathetic function.
   • The parasympathetic system causes pupil constriction (miosis) via the oculomotor nerve (CN III).

3. Relaxation of internal anal sphincter

   • ❌ Incorrect because the internal anal sphincter is primarily controlled by the autonomic nervous system.
   • Sympathetic stimulation keeps it contracted, while parasympathetic stimulation allows relaxation only when defecation is voluntarily initiated (not a direct effect).

4. Tachycardia (Increased heart rate)

   • ❌ Incorrect because tachycardia (increased heart rate) is a sympathetic effect.
   • The parasympathetic system decreases heart rate via the vagus nerve (CN X) by acting on M2 muscarinic receptors in the heart.

Provitamin A is found in vegetables in the form of beta-carotene, a type of carotenoid. The body converts beta-carotene into retinol (active vitamin A) in the intestine and liver as needed. Beta-carotene is present in yellow, orange, and dark green vegetables, such as carrots, sweet potatoes, and spinach.

Why the Other Options Are Wrong:

1. Retinol:
   • Retinol is the active form of vitamin A, found primarily in animal sources such as liver, eggs, and dairy products. It is not the provitamin form found in vegetables.
2. Calciferol (Vitamin D2):
   • Calciferol is Vitamin D2, not related to vitamin A. It is obtained from plant sources and is involved in calcium metabolism.
3. 1,25-Dihydroxycholecalciferol:
   • This is the active form of Vitamin D3 (also called calcitriol). It plays a role in calcium and phosphate homeostasis but has no connection to provitamin A.
4. Alpha-tocopherol:
   • Alpha-tocopherol is Vitamin E, which is an antioxidant that protects cell membranes. It does not have any relation to Vitamin A metabolism.

The answer is Cysteine.

Explanation:

Cysteine is a unique amino acid because it contains a sulfhydryl (-SH) group. This sulfur atom allows cysteine to form disulfide bonds with other cysteine residues, which are important for protein folding and stability.  

Why the other options are incorrect:

• Alanine, serine, tyrosine, and glycine are all standard amino acids found in proteins, but they do not contain sulfur. They have different side chains with other chemical properties.

Intrinsic activity refers to the ability of a drug to elicit a response after binding to its receptor. It is a measure of how well a drug activates the receptor to produce a biological effect.

• Full agonists have high intrinsic activity (near 1), meaning they fully activate the receptor.
• Partial agonists have moderate intrinsic activity (between 0 and 1), meaning they activate the receptor but produce a submaximal response.
• Antagonists have zero intrinsic activity because they bind to the receptor but do not activate it—instead, they block the action of agonists.

Intrinsic activity is closely related to efficacy, but while efficacy refers to the maximum possible response a drug can produce, intrinsic activity is a more fundamental measure of how well a drug activates the receptor once bound.

Why the Other Options Are Wrong:

1. Potency ❌

   • Potency refers to how much drug is needed to produce an effect.
   • It is related to drug concentration and dose, but it does not describe the drug’s ability to elicit a response once bound.

2. Efficacy ❌

   • Efficacy refers to the maximum effect a drug can produce, but it is not the same as intrinsic activity.
   • A drug with low intrinsic activity can still have high efficacy if given in high enough doses.

3. Pharmacology ❌

   • Pharmacology is the broad study of drugs and their effects, not a specific term for receptor activation.

4. Elimination ❌

   • Elimination refers to how a drug is removed from the body, usually via metabolism (liver) or excretion (kidneys).
   • It does not relate to receptor activation.

Spermatogenesis, the process of sperm cell production, occurs in the seminiferous tubules of the testes and is regulated by a complex hormonal interplay involving the hypothalamic-pituitary-gonadal axis.

1. Follicle-stimulating hormone (FSH) – Acts on Sertoli cells within the seminiferous tubules to stimulate sperm maturation. Sertoli cells provide structural and nutritional support to developing sperm and produce inhibin, which negatively regulates FSH secretion.
2. Testosterone – Produced by Leydig cells in response to luteinizing hormone (LH). It is essential for the proliferation and differentiation of spermatogonia into mature sperm. Testosterone binds to androgen receptors on Sertoli cells, facilitating spermatogenesis.

Thus, FSH promotes the process, while testosterone maintains and supports it.

Why the Other Options Are Wrong:

1. Androgens and estrogen –
   • Androgens (Testosterone) play a key role in spermatogenesis, but estrogen has a minimal role in regulating male reproductive function. Although small amounts of estrogen are produced by Sertoli cells, they do not directly regulate spermatogenesis.
   • Incorrect because estrogen is not a primary regulator.
2. Progesterone and luteinizing hormone (LH) –
   • LH stimulates Leydig cells to produce testosterone, which indirectly influences spermatogenesis. However, LH does not directly act on seminiferous tubules.
   • Progesterone is not a major hormone in male reproductive physiology.
   • Incorrect because LH indirectly supports spermatogenesis, and progesterone has no significant role.
3. Testosterone and estrogen –
   • Testosterone is essential, but estrogen does not directly regulate spermatogenesis.
   • Incorrect because it lacks FSH, which is critical for initiating and maintaining spermatogenesis.
4. Progesterone and estrogen –
   • Neither progesterone nor estrogen is a primary regulator of spermatogenesis in males.
   • Incorrect because both hormones have little to no direct involvement in sperm production.

Correct Answer: Potassium (K⁺)

Explanation:
The resting membrane potential of a cell is primarily determined by the movement of ions across the cell membrane. Potassium (K⁺) is the key ion responsible for maintaining the negative charge inside the cell. Here’s why:

1. Resting Membrane Potential: At rest, the cell membrane is more permeable to K⁺ than to other ions due to the presence of leaky potassium channels.
2. Potassium Gradient: There is a high concentration of K⁺ inside the cell and a low concentration outside. Due to the concentration gradient, K⁺ tends to diffuse out of the cell.
3. Effect on Membrane Potential: As K⁺ leaves the cell, it takes positive charges with it, making the inside of the cell more negative relative to the outside. This contributes to the resting membrane potential of approximately -70 mV.

Thus, increased permeability to K⁺ makes the inside of the cell more negative.

Why the Other Options Are Wrong:

1. Chloride (Cl⁻):
   • Chloride is a negatively charged ion. If the membrane becomes more permeable to Cl⁻, it would either enter the cell or leave depending on the electrochemical gradient. However, its movement does not directly contribute to making the inside of the cell more negative. In fact, Cl⁻ influx could make the cell less negative (more depolarized).
2. Sodium (Na⁺):
   • Sodium is a positively charged ion. Increased permeability to Na⁺ would cause it to rush into the cell due to its concentration gradient (high outside, low inside). This influx of positive charges would make the inside of the cell less negative (more depolarized), not more negative.
3. Calcium (Ca²⁺):
   • Calcium is also a positively charged ion. Like Na⁺, increased permeability to Ca²⁺ would cause it to enter the cell, making the inside less negative (more depolarized).
4. Hydrogen (H⁺):
   • Hydrogen ions are positively charged. Increased permeability to H⁺ would cause it to move into the cell, making the inside less negative (more depolarized).

Methionine is one of the two sulfur-containing amino acids, the other being cysteine. It contains a thioether (-S-) group in its side chain, which is important for methyl group transfers in biochemical reactions (e.g., S-adenosylmethionine, SAM). Methionine is also an essential amino acid, meaning it must be obtained from the diet.

Why the Other Options Are Wrong:

1. Proline:
   • Proline is a cyclic imino acid, not a sulfur-containing amino acid. It plays a structural role in proteins by disrupting α-helices and contributing to collagen stability.
2. Glycine:
   • Glycine is the simplest amino acid, with a single hydrogen atom as its side chain. It does not contain sulfur and is important for protein flexibility.
3. Serine:
   • Serine contains a hydroxyl (-OH) group, not sulfur. It plays a role in enzyme active sites and phosphorylation reactions.
4. Arginine:
   • Arginine has a guanidinium group, making it a basic amino acid, but it does not contain sulfur. It is important for the urea cycle and nitric oxide (NO) production.

Ultrasonography is a non-invasive imaging technique used to monitor the development of the embryo and fetus. It uses high-frequency sound waves to create real-time images of the developing baby inside the uterus. Since it does not require penetration of the uterus or extraction of fetal cells, it is considered completely non-invasive and is widely used for assessing fetal growth, detecting congenital anomalies, and monitoring pregnancy progression.

Why the Other Options Are Incorrect

• Chorionic Villus Sampling (CVS):
  This is an invasive procedure where a sample of chorionic villi is taken from the placenta to test for genetic disorders. It involves inserting a catheter or needle into the uterus, making it an invasive technique.

• Pre-implantation Genetic Diagnosis (PGD):
  PGD is performed on embryos before implantation during in-vitro fertilization (IVF). Since it involves biopsy of the embryo, it is an invasive procedure.

• Amniocentesis:
  This is an invasive technique where a needle is inserted into the amniotic sac to collect amniotic fluid for genetic testing and fetal health assessment.

• Cordocentesis (Percutaneous Umbilical Blood Sampling, PUBS):
  This is a highly invasive procedure in which fetal blood is drawn from the umbilical cord to detect blood disorders and infections.

The answer is Proline.

Proline’s unique cyclic structure, with its side chain bonded back to the nitrogen atom, introduces a kink or bend in the polypeptide chain. This rigidity disrupts the regular helical structure commonly found in proteins. It’s often found at the turns of helices or in regions where the protein needs to bend.  

The other amino acids listed (histidine, alanine, valine, and phenylalanine) do not have this ring structure and don’t cause the same disruption to the alpha-helix.

Osmosis is the movement of water molecules across a semi-permeable membrane from a region of low solute concentration (high water potential) to a region of high solute concentration (low water potential). This process occurs passively, meaning it does not require energy (ATP) and continues until equilibrium is reached.

Why the Other Options Are Wrong:

1. Facilitated Diffusion → ❌ Wrong
   • Facilitated diffusion involves the movement of molecules (e.g., glucose, ions) across a membrane through a protein channel or carrier, but it does not apply to water specifically. Water moves by osmosis, not facilitated diffusion.
2. Active Transport → ❌ Wrong
   • Active transport moves substances against their concentration gradient (low to high concentration) and requires ATP. Osmosis, on the other hand, is passive and follows the concentration gradient.
3. Diffusion → ❌ Wrong
   • Diffusion is the movement of solutes (not water) from an area of high concentration to low concentration. Osmosis specifically refers to water movement.
4. Endocytosis → ❌ Wrong
   • Endocytosis is a form of bulk transport where cells engulf molecules into vesicles. It does not describe water movement across a membrane.

The acrosome reaction is a critical step in fertilization, allowing sperm to penetrate the zona pellucida (the glycoprotein layer surrounding the oocyte). Acrosin is a serine protease stored in the acrosome of the sperm and is released during this reaction. It helps digest the zona pellucida, facilitating sperm entry into the egg.

Why the Other Options Are Incorrect:

1. Esterase – Although esterases play a role in various metabolic processes, they are not directly involved in the acrosome reaction. Their primary function is breaking down ester bonds in lipids.

2. Gastrin – This is a hormone involved in stimulating gastric acid secretion in the stomach, completely unrelated to sperm function or fertilization.

3. Peptidase – Peptidases break down peptides into amino acids, but acrosin is the specific enzyme responsible for zona pellucida digestion during fertilization.

4. Glycosidases – These enzymes degrade carbohydrates and may play minor roles in modifying the zona pellucida, but they do not have the primary function of facilitating sperm penetration like acrosin does.

Histiocytes are macrophages found in connective tissue. They originate from monocytes in the blood and differentiate into tissue-specific macrophages. Their primary functions include:

• Phagocytosis of pathogens, dead cells, and debris.
• Antigen presentation to activate immune responses.
• Cytokine secretion to regulate inflammation.

Histiocytes are part of the mononuclear phagocyte system (MPS), along with other tissue-specific macrophages.

Why the Other Options Are Wrong:

1. Kupffer Cell → ❌ Wrong
   • Kupffer cells are macrophages of the liver, found within the sinusoids of the liver.
   • They help in removing bacteria, old red blood cells, and toxins from the blood.
2. Langerhans Cell → ❌ Wrong
   • Langerhans cells are dendritic antigen-presenting cells (APCs) in the skin.
   • They are part of the immune system, helping in pathogen detection and immune response activation.
3. Plasma Cell → ❌ Wrong
   • Plasma cells are mature B lymphocytes that produce antibodies.
   • They are found in lymphoid tissues, bone marrow, and connective tissue but are not macrophages.
4. Dust Cell → ❌ Wrong
   • Dust cells are alveolar macrophages in the lungs.
   • They help in removing inhaled particles, pathogens, and debris from the alveoli.

Proteoglycans are proteins that are covalently bonded to glycosaminoglycans (GAGs), which are long, unbranched polysaccharides composed of repeating disaccharide units. These molecules are essential components of the extracellular matrix (ECM) and connective tissues, providing structural support, lubrication, and hydration.

• The protein core of a proteoglycan is synthesized in the rough endoplasmic reticulum (ER).
• GAG chains (such as chondroitin sulfate, heparan sulfate, and keratan sulfate) are attached to the protein core via serine residues in the Golgi apparatus.
• This covalent linkage occurs through a xylose-galactose-galactose linkage to the hydroxyl group of serine.

Proteoglycans are essential in cartilage, joints, and the basement membrane, where they help retain water and provide resilience to tissues.

Why the Other Options Are Wrong:

1. Amino acid – (Too Generic, Does Not Define GAG Attachment)

   • While proteoglycans do contain amino acids in their protein core, the question asks what they are covalently bonded to, which is glycosaminoglycans (GAGs).

2. Amino glycans – (Incorrect Term, No Such Structure Exists)

   • The term “amino glycans” is not a recognized biological term.
   • The correct term for polysaccharides involved in proteoglycans is glycosaminoglycans (GAGs).

3. Hydroxyl lysine – (Not a Major Attachment Site for GAGs, Used in Collagen Cross-Linking)

   • Hydroxylysine is primarily involved in collagen cross-linking, where it plays a role in stabilizing the collagen fibrils.
   • It does not serve as a site for glycosaminoglycan attachment in proteoglycans.

4. Carbohydrate residues – (Too Broad, Does Not Specify GAGs)

   • While GAGs are a type of carbohydrate, this option is too general.
   • Proteoglycans specifically bind glycosaminoglycans (GAGs), which are distinct from simple carbohydrate residues.

The outer layer of an entire muscle is called the epimysium. It is a dense layer of connective tissue that surrounds the whole muscle, providing protection and support, as well as a surface for the attachment of tendons.

• Endosteum: This is the thin vascular membrane that lines the inner surface of bones, not muscles.
• Periosteum: This fibrous membrane covers the outer surface of bones.
• Perimysium: This connective tissue wraps around bundles (fascicles) of muscle fibers, not the entire muscle.
• Endomysium: This delicate connective tissue layer surrounds each individual muscle fiber.

Tendons are composed of dense regular connective tissue, which is characterized by:

• Parallel bundles of collagen fibers that provide high tensile strength.
• Fibroblasts with flattened nuclei, located between collagen fibers.
• Minimal ground substance and vascularization, making tendons slow to heal.

Tendons connect muscles to bones and allow for efficient force transmission during movement.

Why the Other Options Are Wrong:

1. Aponeurosis → ❌ Wrong
   • Aponeuroses are broad, sheet-like tendons made of dense regular connective tissue, similar to tendons.
   • However, they are flattened and wider, unlike the typical rope-like structure of tendons.
2. Ligament → ❌ Wrong
   • Ligaments also contain dense regular connective tissue, but they have more elastin fibers, making them more flexible than tendons.
   • Tendons are primarily collagen-dominant, whereas ligaments allow some stretch.
3. Adipose Tissue → ❌ Wrong
   • Adipose tissue is a loose connective tissue composed of adipocytes (fat cells), not parallel collagen fibers.
4. Capsule → ❌ Wrong
   • Capsules (e.g., joint capsules, organ capsules) are made of dense irregular connective tissue, where collagen fibers are randomly arranged to resist forces in multiple directions.
   • Tendons, in contrast, have collagen fibers arranged in parallel.

Isomers are compounds with the same molecular formula but different structural arrangements or spatial orientations. Types of isomers include:

1. Structural isomers: Differ in the connectivity of atoms.
2. Stereoisomers: Differ in the spatial arrangement of atoms (e.g., enantiomers, diastereomers).

Other terms:

• Isotopes: Atoms of the same element with different numbers of neutrons.
• Anomer: A type of stereoisomer in carbohydrates, differing at the anomeric carbon.
• Isopod: A type of crustacean, unrelated to chemistry.
• Asymmetric: Refers to a lack of symmetry, not a type of molecule.