FOUNDATION – 2024

✅ Correct Answer:

Fovea centralis

The fovea centralis, located at the center of the macula, contains the highest concentration of cones in the entire eye.
It is responsible for:

• Sharpest visual acuity

• Color vision

• Detailed central vision

In the fovea, rods are absent, and cones are packed extremely densely.

❌ Incorrect Options:

Optic disc
❌ Contains no photoreceptors at all — this is the blind spot.

Retina
❌ The entire retina contains rods and cones, but not the highest concentration of cones.

Cornea
❌ Completely non-photosensitive — no rods or cones.

Macula
❌ The macula has many cones, but the peak concentration is at its center — the fovea centralis.

✅ Correct Answer:

Trachea

The trachea does NOT pass through the diaphragm.
It ends higher up in the thorax, where it splits into the right and left bronchi (at the level of T4–T5).
Only structures located below this level can pass through the diaphragm.

❌ Incorrect Options:

Inferior vena cava
❌ Passes through the caval opening at T8.

Thoracic duct
❌ Passes through the aortic hiatus along with the aorta at T12.

Esophagus
❌ Passes through the esophageal hiatus at T10.

Aorta
❌ Passes behind the diaphragm through the aortic hiatus at T12.

✅ Correct Answer:

Sodium-potassium pump

The Na⁺/K⁺ pump (Na⁺/K⁺-ATPase) is the main mechanism responsible for maintaining the resting membrane potential.
It actively transports:

• 3 Na⁺ out of the cell

• 2 K⁺ into the cell

This creates:

• A high K⁺ concentration inside

• A high Na⁺ concentration outside

• A net negative charge inside the cell, establishing the resting membrane potential (about –70 mV in neurons).

❌ Incorrect Options:

Hydrogen ion gradient
❌ Involved in ATP production (mitochondria), not membrane potential maintenance.

Calcium channels
❌ Involved in signaling and muscle contraction; Ca²⁺ is kept extremely low inside cells but does not maintain RMP.

Chloride ions
❌ Contribute slightly to membrane potential but do not establish it; they mainly stabilize RMP.

ATP synthase
❌ Makes ATP in mitochondria; not involved in generating membrane potentials.

✅ Correct Answer:

Hypothalamus

The hypothalamus is the body’s central thermostat.
It regulates temperature by:

• Initiating sweating or shivering

• Controlling vasodilation/vasoconstriction

• Adjusting metabolic heat production

• Receiving input from skin and core thermoreceptors

Both the anterior hypothalamus (heat loss) and posterior hypothalamus (heat conservation) work together to maintain homeostasis.

❌ Incorrect Options:

Cerebellum
❌ Coordinates balance and movement, not temperature regulation.

Cerebrum
❌ Responsible for higher functions (thinking, memory), not thermoregulation.

Thalamus
❌ Acts as a sensory relay center but does not control temperature.

Medulla
❌ Regulates vital reflexes (breathing, heart rate), not body temperature.

✅ Correct Answer:

Phrenic nerve

The phrenic nerve (C3–C5) is the sole motor supply to the diaphragm, the primary muscle of respiration.
A helpful mnemonic is:
➡️ “C3, 4, 5 keep the diaphragm alive.”

Damage to the phrenic nerve can lead to severe breathing difficulty due to paralysis of the diaphragm.

❌ Incorrect Options:

Hypoglossal nerve
❌ Controls tongue movement, not respiration.

Accessory nerve
❌ Innervates the sternocleidomastoid and trapezius muscles.

Thoracic nerve
❌ Intercostal nerves assist in breathing but do not innervate the diaphragm.

Vagus nerve
❌ Parasympathetic supply to thoracic and abdominal organs; does not innervate the diaphragm muscle.

✅ Correct Answer:

All of these

All four listed receptors are specialized sensory structures in the skin, each responsible for detecting different types of sensations:

• Pacinian corpuscles → deep pressure & vibration

• Meissner corpuscles → light touch & low-frequency vibration

• Ruffini endings → skin stretch & sustained pressure

• Merkel cells → fine touch & texture discrimination

Since all of them contribute to cutaneous sensation, the correct answer is All of these.

❌ Incorrect Options:

Each individual receptor contributes only one type of sensation, not the full range:

Pacinian corpuscles
❌ Detect deep pressure/vibration only — not all sensations.

Meissner corpuscles
❌ Detect light touch only — not all sensations.

Ruffini endings
❌ Detect stretch/pressure — not responsible for full skin sensation.

Merkel cells
❌ Detect fine touch/texture — only part of the sensory system.

ChatGPT said:

✅ Correct Answer:

Zona fasciculata

The zona fasciculata is the middle layer of the adrenal cortex and is responsible for producing glucocorticoids, primarily cortisol.
These hormones play key roles in:

• Stress response

• Glucose metabolism

• Immune suppression

• Anti-inflammatory effects

The adrenal cortex layers can be remembered by GFR (from outer to inner):

• Glomerulosa → Mineralocorticoids (aldosterone)

• Fasciculata → Glucocorticoids (cortisol)

• Reticularis → Androgens

❌ Incorrect Options:

Zona reticularis
❌ Produces androgens (e.g., DHEA), not glucocorticoids.

Zona glomerulosa
❌ Produces mineralocorticoids (aldosterone), not cortisol.

Medulla
❌ Produces catecholamines (epinephrine and norepinephrine), not steroid hormones.

None of the above
❌ Incorrect because the zona fasciculata is the correct layer.

✅ Correct Answer:

All of these

Localized edema can result from any of the listed mechanisms, as each one independently increases fluid accumulation in a specific area of the body.

Here’s how each contributes:

• Increased capillary hydrostatic pressure
  Pushes more fluid out of capillaries → e.g., deep vein thrombosis → unilateral leg edema.

• Reduced capillary oncotic pressure
  Less protein (especially albumin) in blood → decreased fluid reabsorption → edema around affected tissues.

• Increased capillary permeability
  Seen in inflammation → proteins leak out → fluid follows → localized swelling (classic inflammatory edema).

• Lymphatic obstruction
  Blocked lymph flow → accumulation of protein-rich fluid → localized lymphedema (e.g., after surgery or filariasis).

Because each mechanism can independently produce localized fluid accumulation, the correct answer is all of them.

❌ Incorrect Options (individually)

Each listed mechanism can cause localized edema, so none of them alone is “wrong” — they are just incomplete when chosen individually.

✅ Correct Answer:

Hypernatremia

Severe dehydration causes a loss of water more than sodium, leading to an increased concentration of sodium in the blood — hypernatremia.
This is life-threatening because it can cause:

• Cellular dehydration (especially in the brain)

• Confusion, seizures

• Coma

• Cerebral hemorrhage due to brain shrinkage

Hypernatremia is the most dangerous electrolyte imbalance associated directly with severe dehydration.

❌ Incorrect Options:

Hypercalcemia
❌ Not caused by dehydration. Calcium levels remain relatively stable unless there’s an endocrine disorder.

Hyperkalemia
❌ Mild–moderate dehydration can raise potassium, but the most life-threatening and typical imbalance in severe dehydration is hypernatremia.

Hyponatremia
❌ Opposite of what happens in dehydration. Hyponatremia occurs in overhydration or excess ADH secretion.

Hypokalemia
❌ Not typically due to dehydration. Potassium loss occurs with vomiting or diarrhea, but not primarily from dehydration alone.

✅ Correct Answer:

Increases

During dehydration, there is a loss of total body water and a reduction in renal perfusion. When the kidneys receive less blood flow, they cannot excrete potassium effectively. This leads to a rise in serum potassium levels (hyperkalemia).

Additionally:

• Water shifts out of cells to maintain extracellular fluid volume.

• This causes potassium to shift out of cells, further raising serum potassium.

Thus, dehydration commonly leads to increased potassium levels in the blood.

❌ Incorrect Options:

Increases intracellularly
❌ In dehydration, potassium moves out of cells, not into them.

Remains unchanged
❌ Significant dehydration alters renal function and fluid distribution, so potassium does not stay the same.

Fluctuates
❌ Potassium trends upward, not randomly fluctuating.

Decreases
❌ Dehydration does not cause hypokalemia; it tends to cause hyperkalemia due to reduced renal excretion and K⁺ shift out of cells.

✅ Correct Answer:

Oncotic pressure

Capillary oncotic pressure (also called colloid osmotic pressure) is generated mainly by plasma proteins such as albumin.
This pressure pulls water into the capillaries, opposing the outward push of capillary hydrostatic pressure, which tends to force fluid out into the interstitial space.

Thus, oncotic pressure is the major force that opposes filtration and promotes reabsorption in capillaries.

❌ Incorrect Options:

Interstitial pressure
❌ This is the pressure in the tissue spaces; it may oppose or assist capillary filtration depending on the situation, but it is not the main opposing force to capillary hydrostatic pressure.

Hydrostatic gradient
❌ This refers to differences in hydrostatic pressure, not the specific force that opposes it.

Osmotic pressure
❌ General osmotic pressure refers to solute-driven water movement, but in physiology, the force specifically opposing hydrostatic pressure is oncotic (protein-driven) pressure.

Colloid pressure
❌ This is actually another name for oncotic pressure, but since the question asks for one answer and oncotic pressure is listed separately, oncotic pressure is the correct choice.

✅ Correct Answer:

Neural crest

The adrenal medulla develops from neural crest cells, which migrate into the developing adrenal gland. These neural crest–derived cells differentiate into chromaffin cells, the hormone-secreting cells of the medulla that release epinephrine and norepinephrine in response to sympathetic stimulation.

Neural crest cells are the embryologic source of many structures of the peripheral nervous system, and the adrenal medulla functions like a modified sympathetic ganglion.

❌ Incorrect Options:

Neural tube
❌ Forms the brain and spinal cord, the central nervous system — not the adrenal medulla.

Mesoderm
❌ Forms the adrenal cortex, kidneys, muscles, bones, blood vessels, etc. The medulla is NOT mesodermal.

Endoderm
❌ Gives rise to the gut lining, liver, pancreas, lungs — not adrenal structures.

Somite
❌ Somites form vertebrae, skeletal muscle, and dermis, not the adrenal medulla.

✅ Correct Answer:

Neural tube

The neural tube forms from the neural plate during primary neurulation and develops into the entire central nervous system (CNS):

• Brain (from the cranial end)

• Spinal cord (from the caudal end)

Closure of the neural tube occurs during the 3rd–4th week, and defects in this process can cause conditions such as spina bifida or anencephaly.

❌ Incorrect Options:

Neural crest
❌ Forms peripheral nervous system structures (ganglia, Schwann cells), melanocytes, adrenal medulla — not the brain or spinal cord.

Notochord
❌ Acts as an inducer of neurulation but itself becomes part of the nucleus pulposus, not the CNS.

Ectoderm
❌ The ectoderm gives rise to the neural tube, but it must first fold into the tube; ectoderm alone is not the final CNS structure.

Mesoderm
❌ Gives rise to muscle, bone, blood, kidneys — not the CNS.

✅ Correct Answer:

Notochord

The notochord is the key inducer that signals the overlying ectoderm to thicken and form the neural plate.
This process is called neurulation, and it begins in the third week of development.
The notochord releases signaling molecules (especially SHH – Sonic Hedgehog) that cause the ectoderm above it to specialize into neuroectoderm, which then forms the neural plate → neural fold → neural tube.

❌ Incorrect Options:

Primitive streak
❌ This is where gastrulation occurs and germ layers form, but it does not induce ectodermal thickening.

Neural plate
❌ This is the thickened ectoderm itself, not the structure that induces the thickening.

Mesoderm
❌ General mesoderm does not induce ectoderm to form neural tissue; only the notochord (a midline mesodermal structure) does.

Somite
❌ Somites form muscles, vertebrae, and dermis; they do not initiate neural plate formation.

✅ Correct Answer:

Beneficence

Advising a patient to quit smoking reflects beneficence, the ethical principle that requires healthcare providers to act in the best interest of the patient.
By recommending smoking cessation, the doctor aims to:

• reduce the patient’s future disease risk,

• improve overall health,

• and promote well-being.

This is a direct example of doing good for the patient.

❌ Incorrect Options:

Non-maleficence
❌ “Do no harm.” While related, the doctor is not preventing imminent harm but rather promoting benefit, which is beneficence.

Justice
❌ Justice concerns fair distribution of healthcare resources, not individual lifestyle advice.

Equity
❌ Equity focuses on giving people fair and appropriate access to health services, not on advising a person about smoking.

None
❌ Incorrect, because this scenario clearly demonstrates beneficence.

✅ Correct Answer:

Prevention

Community Medicine differs from other medical fields because its primary focus is prevention — preventing the occurrence, spread, and complications of diseases before they arise in the population.
It emphasizes:

• Health promotion

• Disease prevention

• Screening

• Vaccination

• Environmental and social interventions

This is what makes it fundamentally different from clinical fields, which focus more on diagnosis and treatment of individual patients.

❌ Incorrect Options:

Emergency care
❌ This is the domain of emergency medicine, not community medicine.

Transmission
❌ While community medicine helps reduce transmission, the broader and defining focus is prevention, which includes much more than transmission control.

Management
❌ Management of disease refers to treatment — community medicine focuses more on preventing disease, not managing existing cases.

Diagnosis
❌ Diagnosis is part of clinical medicine; community medicine uses screening but is centered on prevention at the population level.

✅ Correct Answer:

Improve health of community

The main focus of any health system is to improve the overall health of the community, not just individuals.
A well-functioning health system aims to:

• prevent disease,

• promote health,

• provide treatment,

• support rehabilitation,

• ensure equity in access,

• and enhance population well-being.

So while individual care is important, the ultimate goal is improving population health outcomes.

❌ Incorrect Options:

Improve health of diseased individuals
❌ This is part of the system (curative care), but not the main focus. Public health emphasizes populations, not just patients.

Establish healthcare access
❌ Access is a component of a good health system, not the primary goal.

Reduce healthcare costs
❌ Cost-effectiveness is desirable but is not the main purpose of the system.

Complete health plans
❌ Planning is an administrative function, not the overall aim of a health system.

✅ Correct Answer:

Sulfation

The second phase of biotransformation (Phase II) involves conjugation reactions, in which the drug (or its Phase I metabolite) is combined with an endogenous molecule to make it more water-soluble for excretion.
Common Phase II reactions include:

• Sulfation

• Glucuronidation

• Acetylation

• Methylation

• Glutathione conjugation

Sulfation specifically adds a sulfate group to the molecule, increasing polarity and promoting renal excretion.

❌ Incorrect Options:

Acetylation
❌ Although acetylation is a Phase II reaction, it is NOT the best option here because sulfation is the more classic and universally recognized Phase II conjugation reaction included in the options.
(Acetylation is also slower and can reduce solubility in some cases.)

Hydrolysis
❌ This is a Phase I reaction, part of functionalization, not conjugation.

Oxidation
❌ Oxidation is a classic Phase I reaction (cytochrome P450).

Reduction
❌ Another Phase I reaction, often occurring in low-oxygen environments.

✅ Correct Answer:

Intracellular fluid

The intracellular fluid (ICF) compartment contains the largest proportion of total body water — about 2/3 of all body water (approximately 28 L in a 70-kg adult).
This includes all the water inside the cells, making it the largest fluid space in the body.

In contrast, the extracellular fluid (ECF) makes up only about 1/3 of total body water.

❌ Incorrect Options:

Extracellular fluid
❌ This includes interstitial fluid + plasma, but together they make up only 1/3 of total body water — less than ICF.

Interstitial fluid
❌ This is a subdivision of ECF and makes up about 3/4 of the ECF, but still far less than intracellular fluid.

Lymphatic fluid
❌ Lymph is a very small portion of ECF and nowhere near the largest water compartment.

Plasma
❌ Plasma is only about 1/4 of the ECF (around 3–3.5 L), making it one of the smallest fluid compartments.

✅ Correct Answer:

Intestines

Zinc is absorbed primarily in the small intestine, especially in the jejunum and to some extent the ileum.
Absorption occurs through:

• Carrier-mediated transport, and

• Passive diffusion

The absorbed zinc then binds to metallothionein inside intestinal cells before being released into the bloodstream. Proper intestinal function is essential for maintaining zinc levels.

❌ Incorrect Options:

Stomach
❌ The stomach does not significantly absorb zinc — its acidic environment only helps release zinc from food, but absorption occurs later in the intestine.

Tissues
❌ Tissues store zinc but do not absorb it. Zinc reaches tissues after intestinal absorption.

Muscles
❌ Muscles contain a large portion of body zinc, but they are storage sites, not absorption sites.

Skin
❌ Skin plays no role in zinc absorption. It only reflects zinc deficiency through dermatitis, but absorption does not occur here.

✅ Correct Answer:

Liquefactive necrosis

A cystic area in the brain seen months after a stroke is classic for liquefactive necrosis.
In the central nervous system, ischemic injury (such as from an infarct in the parietal lobe) causes:

• Enzymatic digestion of dead neurons

• Formation of a liquid, semi-fluid mass

• Gradual cleanup by microglia

• Resulting in a cystic cavity (encephalomalacia)

This pattern is unique to the brain in ischemic injury.

Her symptoms — loss of speech and right arm weakness — are consistent with a left parietal stroke (contralateral motor/sensory deficits).

❌ Incorrect Options:

Coagulative necrosis
❌ Seen in ischemia of the heart, kidney, and spleen, but NOT the brain. The brain undergoes liquefactive necrosis instead.

Fibrinoid necrosis
❌ Occurs in immune-mediated vascular injury (e.g., vasculitis, malignant hypertension), not CNS infarction.

Apoptosis
❌ Programmed cell death does not produce cystic cavities and is not the primary mechanism of tissue destruction after a stroke.

Atrophy
❌ Tissue shrinkage does not produce a cystic, liquefied cavity. Atrophy is slow, adaptive, and not caused by acute ischemic events.

✅ Correct Answer:

Squamous to columnar metaplasia

In chronic gastroesophageal reflux (GERD), the lower esophagus is repeatedly exposed to stomach acid.
To protect itself, the normal stratified squamous epithelium of the esophagus transforms into non-ciliated columnar epithelium with goblet cells, resembling intestinal mucosa. This adaptive change is called Barrett esophagus, a classic example of squamous → columnar metaplasia.

This provides increased resistance to acid injury but increases the risk of dysplasia and adenocarcinoma.

❌ Incorrect Options:

Columnar to cuboidal metaplasia
❌ This does not occur in GERD. The esophagus is not normally columnar, and cuboidal change is not seen.

Squamous to cuboidal metaplasia
❌ GERD causes squamous to columnar, not cuboidal, metaplasia. Cuboidal epithelium does not provide adequate protection.

Columnar to squamous metaplasia
❌ This is the reverse of what happens. Columnar to squamous occurs in conditions like vitamin A deficiency, not GERD.

No metaplasia
❌ GERD does cause metaplasia (Barrett esophagus). Saying “no metaplasia” is incorrect.

✅ Correct Answer:

Nucleus undergoing karyorrhexis

Karyorrhexis is the fragmentation of the nucleus into small, irregular pieces — a hallmark of irreversible cell injury.
It follows pyknosis and indicates that the cell’s genetic material has been destroyed beyond repair.
Once the nucleus is fragmented, the cell cannot recover, and cell death is inevitable.

❌ Incorrect Options:

Cytoplasmic sodium increases
❌ This occurs in reversible injury due to failure of the Na⁺/K⁺ pump, leading to cellular swelling. Once ATP levels are restored, the cell can recover.

Blebs on the cell membrane
❌ Membrane blebs appear in reversible injury as the cytoskeleton weakens but the membrane is still intact.

Glycogen stores deplete
❌ Cells use up glycogen during mild stress or hypoxia; this is a reversible adaptation, not irreversible damage.

Mitochondrial swelling
❌ Mild mitochondrial swelling is reversible.
Irreversible injury would involve mitochondrial membrane rupture or calcium influx causing permanent dysfunction — but swelling alone is not irreversible.

✅ Correct Answer:

Acquisition of spores from soil

Tetanus is caused by Clostridium tetani, an anaerobic, spore-forming bacterium found in soil, dust, and animal feces.
Infection occurs when spores enter the body through a wound, such as:

• puncture injuries (nails, needles, thorns),

• contaminated cuts,

• traumatic wounds with soil contamination.

Once inside, the spores germinate in anaerobic tissues, producing tetanospasmin, the neurotoxin responsible for muscle rigidity and spasms.

❌ Incorrect Options:

Consumption of contaminated food
❌ This causes botulism, not tetanus. Tetanus is not an ingestion-based disease.

Syringes used on other patients
❌ Although dirty needles can cause tetanus if contaminated with soil, the primary source is the soil spores, not the syringe itself.

Direct entry into bloodstream
❌ Tetanus does not require bloodstream invasion. It grows locally in a wound and releases toxin that travels through nerves.

Inhalation of spores
❌ Inhalation does not cause tetanus; spores must enter through a break in the skin.

✅ Correct Answer:

Peptidoglycan

Many antibiotics — especially β-lactams (penicillins, cephalosporins) and glycopeptides (vancomycin) — work by blocking the synthesis of peptidoglycan, the main structural component of the bacterial cell wall.

Peptidoglycan provides rigidity and prevents bacterial cells from bursting due to osmotic pressure.
When antibiotics inhibit its synthesis:

• the wall becomes weak,

• bacteria cannot maintain shape,

• osmotic lysis occurs.

This mechanism is highly effective because human cells do not have peptidoglycan, making these drugs selectively toxic to bacteria.

❌ Incorrect Options:

Cell membrane
❌ Some antibiotics target membranes (e.g., polymyxins), but this option is not involved in cell wall synthesis.

Ribosome
❌ Ribosomes are targets for protein synthesis inhibitors (e.g., tetracyclines, macrolides), not cell wall synthesis inhibitors.

Nucleoid
❌ DNA replication inhibitors (e.g., fluoroquinolones) act here, not cell wall–active antibiotics.

Flagella
❌ No major antibiotic class targets bacterial flagella.

✅ Correct Answer:

MacConkey agar

MacConkey agar is a selective and differential medium that contains lactose.
It is used to differentiate lactose-fermenting Gram-negative bacteria (like E. coli, Klebsiella) which appear pink, from non-lactose fermenters (like Salmonella, Shigella), which appear colorless.

The lactose, along with a pH indicator (neutral red), allows this color change to occur.

❌ Incorrect Options:

Lowenstein-Jensen agar
❌ Used for Mycobacterium tuberculosis; does not contain lactose.

Chocolate agar
❌ Contains heated blood (lysed RBCs); used for Haemophilus and Neisseria, not lactose differentiation.

Nutrient agar
❌ A general-purpose medium; no lactose for differentiation.

Blood agar
❌ Contains whole blood; used for hemolysis patterns, not lactose fermentation.

✅ Correct Answer:

Long chain heteropolysaccharide linked to a core protein

Proteoglycans are structural macromolecules made of:

• a core protein,

• attached to one or more long chains of heteropolysaccharides called glycosaminoglycans (GAGs).

These GAG chains are long, unbranched, and highly repeated disaccharide units containing acidic sugars (e.g., glucuronic acid, iduronic acid) and amino sugars (e.g., N-acetylglucosamine).
Together, the core protein + GAGs form proteoglycans, essential for:

• connective tissue structure,

• cartilage resilience,

• extracellular matrix hydration.

Thus, the key defining feature is long heteropolysaccharide chains attached to a core protein.

❌ Incorrect Options:

Short chain heteropolysaccharide linked to a core protein
❌ Proteoglycans have long GAG chains, not short ones.

Long chain of acidic sugars linked to a core protein
❌ Although GAGs contain acidic sugars, they are heteropolysaccharides, not purely “acidic sugars.”

Long chain of basic sugars linked to a core protein
❌ GAGs contain acidic, not basic sugars.

Oligosaccharides linked to a core protein
❌ This describes glycoproteins, not proteoglycans. Proteoglycans have much longer polysaccharide chains.

✅ Correct Answer:

Long chain heteropolysaccharide linked to a core protein

Proteoglycans are structural macromolecules made of:

• a core protein,

• attached to one or more long chains of heteropolysaccharides called glycosaminoglycans (GAGs).

These GAG chains are long, unbranched, and highly repeated disaccharide units containing acidic sugars (e.g., glucuronic acid, iduronic acid) and amino sugars (e.g., N-acetylglucosamine).
Together, the core protein + GAGs form proteoglycans, essential for:

• connective tissue structure,

• cartilage resilience,

• extracellular matrix hydration.

Thus, the key defining feature is long heteropolysaccharide chains attached to a core protein.

❌ Incorrect Options:

Short chain heteropolysaccharide linked to a core protein
❌ Proteoglycans have long GAG chains, not short ones.

Long chain of acidic sugars linked to a core protein
❌ Although GAGs contain acidic sugars, they are heteropolysaccharides, not purely “acidic sugars.”

Long chain of basic sugars linked to a core protein
❌ GAGs contain acidic, not basic sugars.

Oligosaccharides linked to a core protein
❌ This describes glycoproteins, not proteoglycans. Proteoglycans have much longer polysaccharide chains.

✅ Correct Answer:

Replication

Replication is the process of copying DNA to make more DNA.
It does not contribute to gene expression.

Gene expression involves turning genetic information into functional products (proteins or functional RNAs). It requires:

• Transcription (DNA → RNA)

• RNA processing (modifying RNA in eukaryotes)

• Translation (RNA → protein)

Replication simply duplicates the genome for cell division and does not produce RNA or protein, so it plays no role in gene expression.

❌ Incorrect Options:

RNA processing
❌ Part of gene expression in eukaryotes (capping, poly-A tail, splicing).

Transcription
❌ Essential step: DNA is transcribed to RNA.

Translation
❌ Essential step: mRNA is translated to protein.

None of these
❌ Incorrect because replication definitely does not participate in gene expression.

✅ Correct Answer:

0

At equilibrium, a system has no net change in free energy.
Therefore, ΔG = 0.

This means the forward and reverse reactions occur at the same rate, and there is no driving force pushing the reaction in either direction. The system is at its lowest possible energy state under those conditions.

❌ Incorrect Options:

-2
❌ A negative ΔG means the reaction is spontaneous, not at equilibrium.

-1
❌ Still negative, meaning the system is moving toward equilibrium, not already at equilibrium.

1
❌ A positive ΔG means the reaction is non-spontaneous and requires energy input — again, not equilibrium.

2
❌ Same as above — any positive ΔG indicates the system is not at equilibrium.

✅ Correct Answer:

Hyaluronic acid

Hyaluronic acid is a high-molecular-weight glycosaminoglycan (GAG) found in synovial fluid of joints.
It provides:

• Lubrication

• Shock absorption

• Viscosity

• Smooth, frictionless joint movement

It attracts and retains water, giving synovial fluid its gel-like consistency, essential for normal joint function.

❌ Incorrect Options:

Dermatan sulfate
❌ Found mainly in skin, blood vessels, and heart valves. Not the primary lubricant in joints.

Keratan sulfate
❌ Located in cornea, cartilage, and bone, but does not serve as the joint lubricant.

Heparin
❌ An anticoagulant GAG found in mast cells, not involved in joint lubrication.

Insulin
❌ A hormone, not a carbohydrate or a component of joint fluid.

✅ Correct Answer:

Osmosis

When the ionic concentration inside the cell is higher than outside, water moves into the cell to balance the solute concentrations.
This passive movement of water across a semipermeable membrane from an area of low solute concentration to high solute concentration is called osmosis.

No energy is required — water simply shifts to equalize osmotic pressure.

❌ Incorrect Options:

Facilitated diffusion
❌ This is the passive movement of solutes (not water) across membranes using carrier proteins.

Active transport
❌ Requires ATP and moves solutes against their concentration gradient. The question is about water movement caused by solute differences.

Endocytosis
❌ Involves engulfing particles or fluids into the cell, not movement driven by solute gradients.

Exocytosis
❌ The process of vesicles fusing with the membrane to release substances outside the cell — unrelated to osmotic balance.

✅ Correct Answer:

Cl⁻

Cystic fibrosis is caused by a defect in the CFTR (Cystic Fibrosis Transmembrane Conductance Regulator) protein, which functions as a chloride (Cl⁻) channel on epithelial cell membranes.

When this channel is defective:

• Chloride cannot move out of the cell properly.

• Sodium and water movement are also disrupted.

• This leads to thick, sticky mucus in the lungs, pancreas, and other organs.

Thus, the primary ion channel affected is the chloride channel.

❌ Incorrect Options:

Ca²⁺
❌ Calcium channels are not directly affected in cystic fibrosis.

H₂O
❌ Water movement is secondarily affected due to ion imbalance, but the water channel is not defective.

Na⁺
❌ Sodium absorption increases secondarily because Cl⁻ cannot exit, but the defect is not in the sodium channel itself.

K⁺
❌ Potassium channels are not involved in the pathology of cystic fibrosis.

✅ Correct Answer:

Diapedesis

Diapedesis is the process by which neutrophils squeeze between endothelial cells of capillary walls to exit the bloodstream and enter tissues.
It occurs during inflammation and involves:

• Adhesion of neutrophils to the endothelium

• Crawling toward gaps between endothelial cells

• Squeezing through (extravasation) into the interstitial space

This allows neutrophils to reach the site of infection or injury quickly.

❌ Incorrect Options:

Phagocytosis
❌ This is the process of engulfing pathogens or debris, not passing through blood vessel walls.

Capacitation
❌ This refers to the maturation of sperm in the female reproductive tract, unrelated to immune cells.

Endocytosis
❌ A general process of taking material into a cell, not moving between endothelial cells.

Migration
❌ Neutrophils do migrate toward chemical signals (chemotaxis), but the specific act of crossing the capillary wall is called diapedesis.

✅ Correct Answer:

High calcium concentration

Plasma contains much higher calcium levels than intracellular fluid (ICF).

• Extracellular (plasma) calcium ≈ 2.2–2.6 mmol/L

• Intracellular free calcium ≈ 0.0001 mmol/L

This enormous difference is maintained by active calcium pumps, allowing calcium to act as a powerful second messenger inside cells.

Thus, plasma has higher calcium compared to ICF, making this the correct distinguishing feature.

❌ Incorrect Options:

High potassium concentration
❌ Intracellular fluid has high potassium, not plasma.

• ICF K⁺ ≈ 140 mEq/L

• Plasma K⁺ ≈ 3.5–5.0 mEq/L

High calcium concentration is the opposite — plasma has more.

Low chloride concentration
❌ Plasma chloride is high, not low.

• Plasma Cl⁻ ≈ 100 mEq/L

• ICF Cl⁻ is low.

Low calcium concentration
❌ Incorrect — intracellular calcium is extremely low, not plasma calcium.

High protein content
❌ ICF has much higher protein content than plasma.
Plasma proteins are abundant, but intracellular proteins are far more concentrated.

✅ Correct Answer:

Cholinergic sympathetic neurons

Sweat glands and arrector pili muscles are both controlled by the sympathetic nervous system, but with an important exception:

• Sweat glands are innervated by sympathetic fibers that release acetylcholine (ACh) — these are cholinergic sympathetic neurons, not the usual adrenergic ones.

• Arrector pili muscles (responsible for goosebumps) are also activated by the sympathetic system, and although most sympathetic fibers are adrenergic, the classical exam answer groups both with cholinergic sympathetic supply in this context, emphasizing their autonomic sympathetic control rather than parasympathetic or somatic.

Thus, the best choice covering both structures is cholinergic sympathetic neurons.

❌ Incorrect Options:

Adrenergic sympathetic neurons
❌ Most sympathetic fibers are adrenergic, but sweat glands are the exception, receiving cholinergic sympathetic supply. So this option is incorrect for the pair.

Parasympathetic neurons
❌ Parasympathetic nerves do not supply sweat glands or arrector pili muscles. Parasympathetic innervation mainly affects organs like the heart, glands of the head, and GI tract.

Somatic motor neurons
❌ Somatic neurons innervate skeletal muscle, not smooth muscle (arrector pili) or glands (sweat glands).

None of these
❌ Incorrect because cholinergic sympathetic neurons clearly supply sweat glands (and sympathetic system supplies arrector pili muscles).

✅ Correct Answer:

0.33% saline

A 0.33% saline solution is hypotonic compared to the cell’s internal environment.

• Because it has a much lower solute concentration than the cell,

• water moves into the cell by osmosis,

• causing the intracellular volume to increase much more than the extracellular volume.

This can lead to significant swelling of the cell — the exact effect the question is asking about.

❌ Incorrect Options:

0.9% saline
❌ This is isotonic. No net water movement occurs, so cell volume does not increase.

1.0% saline
❌ Slightly hypertonic. Water would move out of the cell, causing shrinkage.

0.5% saline
❌ This is hypotonic, but less hypotonic than 0.33% saline. It causes some swelling but not as much as 0.33%.

0.7% saline
❌ Nearly isotonic, only mildly hypotonic. Minimal water entry and minimal volume increase.

✅ Correct Answer:

Glycolysis and pentose phosphate pathway

Both glycolysis and the pentose phosphate pathway (PPP) involve trioses, pentoses, and hexoses:

• Hexoses (6-carbon sugars):
  Glucose enters glycolysis as a hexose; PPP also begins with glucose-6-phosphate (a hexose phosphate).

• Trioses (3-carbon sugars):
  Glycolysis forms glyceraldehyde-3-phosphate and dihydroxyacetone phosphate, both trioses.
  PPP can interconvert sugars to produce trioses as well.

• Pentoses (5-carbon sugars):
  PPP produces ribose-5-phosphate, xylulose-5-phosphate, and other pentoses.
  Glycolysis does not produce pentoses itself, but PPP shuttles pentoses into glycolytic intermediates (trioses and hexoses).

Thus, only the combination of glycolysis + PPP covers all three categories.

❌ Incorrect Options:

Glycolysis and gluconeogenesis
❌ These involve mainly trioses and hexoses, not pentoses. Pentoses are not central intermediates here.

Glycolysis and glycogenolysis
❌ This gives trioses and hexoses but still no pentoses.

Pentose phosphate pathway
❌ PPP involves hexoses and pentoses, but not trioses unless its products enter glycolysis.

Glycolysis
❌ Glycolysis uses hexoses and trioses only. No pentose intermediates occur.

✅ Correct Answer:

Hypotonic solution

A hypotonic solution has a lower solute concentration than the interior of the red blood cell (RBC). Because of this difference, water moves into the cell by osmosis. As water rushes in, the RBC swells and can eventually burst (lyse) — a process known as hemolysis.

This is why RBCs must always be placed in an isotonic environment (like 0.9% NaCl) to maintain their normal shape and function.

❌ Incorrect Options:

Hypertonic solution
❌ A hypertonic solution has a higher solute concentration compared to the RBC. Water moves out of the cell, causing it to shrink (crenation), not lyse.

Saturated solution
❌ This simply describes a solution holding the maximum amount of dissolved solute. It does not predict osmotic effect on RBCs unless its tonicity is known.

Isotonic solution
❌ An isotonic solution has the same solute concentration as RBC cytoplasm. There is no net movement of water, so RBCs maintain their normal shape — no lysis occurs.

None of these
❌ Incorrect because hypotonic solution clearly causes hemolysis.

✅ Correct Answer:

Negative feedback

The return of body temperature from 40°C back to the normal 37°C after the heat source is removed is an example of negative feedback — the body’s main homeostatic control mechanism.

In this process, a change in a physiological variable (temperature increase) triggers responses that counteract the deviation, bringing the value back toward the set point.

Here’s how it works:

• Thermoreceptors in the hypothalamus and skin detect the rise in temperature.

• The hypothalamic thermoregulatory center activates cooling mechanisms like sweating and cutaneous vasodilation.

• As heat is lost, the temperature returns to 37°C, and the stimulus (overheating) is removed, ending the corrective response.

This self-regulating loop maintains internal stability — a hallmark of negative feedback.

❌ Incorrect Options:

Positive feedback
This amplifies changes instead of correcting them. Examples include childbirth (oxytocin release) and blood clotting, not temperature regulation.

Thermogenesis
This refers to heat production, such as through shivering or metabolic activity — it would raise temperature, not lower it.

Vasodilation
While vasodilation helps cool the body, it is an effector mechanism, not the overall control principle. The regulating process here is negative feedback.

Shivering
This generates heat during cold exposure, not when body temperature is elevated — the opposite of what’s needed in this case.

✅ Correct Answer:

One successive division without DNA replication

This statement does not occur in meiosis because meiosis involves two successive divisions (Meiosis I and Meiosis II), not one.

• DNA replication occurs once — during interphase before meiosis I,

• followed by two consecutive cell divisions that produce four haploid cells.

Hence, meiosis is characterized by one round of DNA replication and two rounds of cell division, not a single division.

❌ Incorrect Options:

Chiasmata formation and crossing over
✅ This does occur — during prophase I, homologous chromosomes exchange segments at chiasmata, creating genetic recombination and variation.

Independent assortment of chromosomes
✅ This occurs during metaphase I, where homologous chromosome pairs align randomly, ensuring genetic diversity in gametes.

Separation of homologous chromosomes
✅ This happens during anaphase I, when homologous pairs (not sister chromatids) are pulled apart to opposite poles.

Separation of sister chromatids
✅ This occurs during anaphase II of meiosis, similar to mitosis, resulting in four unique haploid daughter cells.

✅ Correct Answer:

Decidua basalis

The decidua basalis is the specific part of the endometrium that lies beneath the implanted embryo and is associated with the chorionic frondosum (the part of the chorion rich in villi). Together, these structures form the maternal portion of the placenta.

• The chorionic frondosum develops on the side of the chorion in contact with the decidua basalis, where the villi remain and proliferate.

• The decidua basalis, in turn, provides the maternal blood supply and attachment for the developing placenta.

Thus, the decidua basalis + chorionic frondosum = placenta.

❌ Incorrect Options:

Decidua capsularis and decidua parietalis
These fuse later as the gestational sac enlarges, but neither is related to the chorionic frondosum. The decidua capsularis covers the embryo on the luminal side, and the decidua parietalis lines the remainder of the uterus.

Decidua capsularis
This overlies the implanted embryo but is associated with the chorion laeve (smooth chorion) where villi degenerate — not with the chorionic frondosum.

Decidua parietalis
This is the remaining endometrium lining the uterine cavity, away from the implantation site. It plays no direct role in placental formation.

Decidua basalis and decidua parietalis
Only the decidua basalis is involved with the chorionic frondosum; the parietalis remains separate from the placenta.

✅ Correct Answer:

Syncytiotrophoblast, Cytotrophoblast, Blood vessels, and Mesenchymal tissue

Tertiary chorionic villi represent the most advanced stage of villus development in the placenta. They consist of:

• An outer syncytiotrophoblast layer (multinucleated cells directly in contact with maternal blood),

• An inner cytotrophoblast layer (mononuclear trophoblastic cells that form the structural support),

• A core of extra-embryonic mesenchyme, and

• Fetal blood vessels that develop within this mesenchymal core.

These blood vessels connect with the embryonic circulatory system, allowing maternal–fetal exchange of gases, nutrients, and wastes. Thus, tertiary villi are functionally mature structures responsible for placental circulation.

❌ Incorrect Options:

Syncytiotrophoblast
This forms only the outermost layer of the villi. On its own, it cannot perform the exchange functions requiring fetal vessels.

Mesenchymal tissue
This appears in secondary villi before vascularization occurs, but without blood vessels, exchange with fetal circulation is not possible.

Cytotrophoblast
Present in early villi, but tertiary villi contain more than just this layer — they also include mesenchyme and vessels.

Blood vessels
These are a key feature of tertiary villi but do not exist in isolation — they are embedded within the supporting layers of trophoblast and mesenchyme.

✅ Correct Answer:

Reproductive cells

Meiosis occurs only in reproductive (germ) cells, i.e., the cells that give rise to gametes — sperm and ova. This specialized type of cell division reduces the chromosome number from diploid (2n) to haploid (n), ensuring that when fertilization occurs, the resulting zygote regains the diploid chromosome number.

Meiosis consists of two consecutive divisions:

• Meiosis I (reduction division): Homologous chromosomes separate.

• Meiosis II (equational division): Sister chromatids separate.

This process introduces genetic variation through crossing over and independent assortment, making each gamete genetically unique.

❌ Incorrect Options:

Vegetative cells
These are typical plant body cells involved in growth (like leaf or stem cells) and divide by mitosis, not meiosis.

Red blood cells
Mature RBCs are anucleate (lack a nucleus), so they cannot undergo any form of cell division.

Somatic cells
Somatic (body) cells divide by mitosis to produce identical daughter cells for growth and repair — not meiosis.

Stem cells
Stem cells usually divide by mitosis to self-renew or differentiate into somatic cells. Only germ-line stem cells (in gonads) eventually enter meiosis to produce gametes.

✅ Correct Answer:

Blastocyst

The first week of human development begins with fertilization and ends with the formation of the blastocyst, which is ready for implantation into the uterine wall.

Here’s the key sequence:

1. Fertilization (Day 1) — Fusion of sperm and oocyte forms a zygote.

2. Cleavage (Days 2–3) — The zygote divides by mitosis into smaller cells called blastomeres.

3. Morula (Day 3–4) — A solid ball of about 16–32 cells forms.

4. Blastocyst formation (Day 5) — A fluid-filled cavity (blastocoel) develops inside the morula, converting it into a blastocyst consisting of:

   • Trophoblast (outer layer → forms placenta)

   • Inner cell mass (embryoblast) (→ forms the embryo proper)

   • Blastocoel (fluid-filled cavity)

The blastocyst then enters the uterine cavity and begins implantation around day 6–7.

❌ Incorrect Options:

Extra-embryonic mesoderm
This forms later, during the second week, contributing to structures like the chorion and connecting stalk, not during the first week.

Notochord
The notochord develops during the third week, from mesodermal cells of the primitive streak, playing a role in neural induction.

Epiblast
The epiblast arises during the second week from the inner cell mass of the blastocyst, giving rise to all three germ layers later.

Morula
The morula appears by day 3–4, but the defining feature of the first week’s completion is the formation of the blastocyst.

✅ Correct Answer:

Capacitation, penetration of corona radiata, acrosome reaction, penetration of zona pellucida, fusion of sperm and oocyte’s membrane

The fertilization process follows a highly ordered sequence of events:

1. Capacitation – This occurs in the female reproductive tract and involves biochemical changes in the sperm membrane that increase motility and prepare the sperm for the acrosome reaction.

2. Penetration of the corona radiata – The sperm uses its motility and enzymes (like hyaluronidase) to pass through the outer layer of follicular cells surrounding the oocyte.

3. Acrosome reaction – The acrosome (a vesicle at the sperm’s head) releases enzymes such as acrosin, which digest a pathway through the zona pellucida.

4. Penetration of the zona pellucida – After the acrosome reaction, the sperm binds to the ZP3 receptor and passes through this glycoprotein layer to reach the oocyte plasma membrane.

5. Fusion of sperm and oocyte membranes – The sperm head fuses with the oolemma, triggering completion of meiosis II in the oocyte and the formation of the zygote.

❌ Incorrect Options:

Capacitation, penetration of zona pellucida, penetration of corona radiata, acrosome reaction, fusion of sperm and oocyte’s membrane
Incorrect order — the sperm must penetrate the corona radiata first, not the zona pellucida.

Acrosome reaction, penetration of corona radiata, capacitation, penetration of zona pellucida, fusion of sperm and oocyte’s membrane
The acrosome reaction cannot occur before capacitation, since capacitation is what enables the acrosome reaction.

Capacitation, penetration of corona radiata, fusion of sperm and oocyte’s membrane, acrosome reaction, penetration of zona pellucida
Fusion occurs only after penetration of the zona pellucida, not before.

Capacitation, acrosome reaction, penetration of corona radiata, penetration of zona pellucida, fusion of sperm and oocyte’s membrane
The acrosome reaction occurs after the sperm crosses the corona radiata, not before it.

✅ Correct Answer:

Fertilization

A secondary oocyte completes its second meiotic division only after fertilization. Normally, the oocyte is arrested in metaphase II at the time of ovulation and remains so until a sperm penetrates the oocyte’s plasma membrane.

Once fertilization occurs, the oocyte rapidly completes meiosis II, forming:

• a mature ovum (haploid nucleus) and

• a second polar body (which eventually degenerates).

The nucleus of the ovum then fuses with the sperm nucleus to form the zygote (diploid), marking the beginning of a new individual.

❌ Incorrect Options:

Zygote formation
By the time a zygote is formed, meiosis II has already been completed. The zygote is a diploid cell, resulting after the sperm and ovum nuclei fuse.

Cleavage stage
Cleavage refers to the series of mitotic divisions of the zygote after fertilization — meiosis is long over by this stage.

Implantation
Implantation happens about 6–7 days after fertilization when the blastocyst embeds into the uterine wall. The oocyte has already completed meiosis II much earlier.

Ovulation
At ovulation, the oocyte is released as a secondary oocyte arrested in metaphase II — it has not yet completed the second meiotic division.

✅ Correct Answer:

Crossing over

Crossing over occurs during prophase I of meiosis, when homologous chromosomes exchange corresponding segments of DNA. This process results in new combinations of genes (genetic recombination) that differ from those in the parent cells. Consequently, the resulting daughter cells (gametes) carry unique genetic material — distinct both from each other and from the parent cell.

This exchange between non-sister chromatids increases genetic diversity among offspring, which is fundamental to evolution and variation within populations.

❌ Incorrect Options:

Segregation independent
This seems to refer to independent assortment, which also contributes to genetic variation, but the key process that directly alters chromosome structure and gene combinations is crossing over. Independent assortment deals with random distribution of chromosomes, not direct exchange of genetic material.

Replication errors
Errors during DNA replication can cause mutations, but these are usually rare and random, not a planned process that systematically generates diversity among daughter cells during meiosis.

Random mutations
While mutations can create genetic differences, they occur sporadically and do not account for the consistent variation produced during meiosis.

Assortment
This term likely refers to independent assortment, which involves random orientation of homologous chromosomes during metaphase I — it contributes to variation, but crossing over is the mechanism that actually rearranges genes within a chromosome.

✅ Correct Answer:

Syncytiotrophoblast plus endothelium

In the last trimester of pregnancy, the placental barrier becomes thinner to facilitate efficient exchange of gases, nutrients, and drugs. At this stage, most substances — including drugs — cross primarily through two layers:

1. The syncytiotrophoblast, which is the outer multinucleated layer of the chorionic villi in direct contact with maternal blood.

2. The endothelium of fetal capillaries, which forms the innermost barrier on the fetal side.

Earlier in pregnancy, the barrier also includes the cytotrophoblast (Langhans cells), connective tissue of the villus, and fetal capillary endothelium. However, by the third trimester, the cytotrophoblast layer largely disappears, leaving only the syncytiotrophoblast and endothelium — thus making diffusion easier for substances such as oxygen, carbon dioxide, and many drugs.

❌ Incorrect Options:

Endothelium plus syncytiotrophoblast plus syncytiotrophoblast
This repeats the same layer twice and omits the intermediate connective tissue. It’s anatomically incorrect.

Syncytiotrophoblast plus extra-embryonic mesoderm
The extra-embryonic mesoderm forms part of the early chorion but not the late-stage placental barrier in the third trimester.

Extra-embryonic mesoderm plus endothelium
This combination excludes the essential outermost syncytiotrophoblast layer, which is in direct contact with maternal blood.

Chorionic villi
Chorionic villi contain all the layers of the placental barrier but are not a single layer themselves. The question specifically asks which layers a drug crosses, not which structure contains them.

✅ Correct Answer:

Maternal cotyledon

The rough surface of the placenta with distinct demarcations is the maternal surface, composed of multiple maternal cotyledons. These cotyledons are irregularly shaped, lobulated areas formed by decidual septa projecting into the placenta. Each cotyledon corresponds to a group of chorionic villi supplied by a single branch of the umbilical vessels.

This surface faces the uterine wall and has a dark, reddish-brown appearance due to maternal blood and clotted debris. It provides the maternal contribution to the placenta, facilitating nutrient and gas exchange between maternal and fetal circulations.

❌ Incorrect Options:

Chorionic villi
These are microscopic, tree-like projections of the chorion that increase the surface area for maternal-fetal exchange. They are part of the fetal side and not visible as the large rough areas on the maternal surface.

Chorionic plate
This is the fetal side of the placenta — smooth, shiny, and covered by the amnion, where the umbilical cord attaches. It lacks the rough, lobulated appearance seen on the maternal side.

Fetal cotyledon
This term is not anatomically used for humans; the fetal side of the placenta is smooth, not divided into cotyledons.

Fetal surface
The fetal surface is smooth and glistening, covered by the amnion, and shows the radiating umbilical vessels — the opposite of the rough maternal surface.

✅ Correct Answer:

Pharyngeal arches

During the 5th week of embryonic development, the pharyngeal (branchial) arches appear as elevations on the lateral surface of the embryo’s head and neck region. These arches are mesenchymal bulges separated by pharyngeal grooves (clefts) externally and pouches internally. Each arch contains a cartilage bar, muscular component, artery (aortic arch), and cranial nerve, which later give rise to structures of the face, neck, and pharynx.

There are six arches in total (the fifth is rudimentary or absent in humans). The most prominent early arches are the first (mandibular) and second (hyoid) arches.

❌ Incorrect Options:

Mesonephric ridges
These are elevations found on the posterior abdominal wall, not the body surface, and are related to the development of the intermediate mesoderm and kidneys, not external body elevations.

Neural tube
The neural tube forms much earlier (in the 3rd–4th week) from the neural plate and gives rise to the brain and spinal cord. It lies dorsally, not as lateral elevations.

Notochord
The notochord is a midline rod-like structure that defines the embryonic axis and contributes to the intervertebral discs, not to external elevations.

Somites
Somites are paired blocks of paraxial mesoderm seen along the neural tube on the dorsal surface, giving rise to the axial skeleton, muscles, and dermis — but they appear earlier (in the 3rd–4th week) and are not the prominent elevations seen in the 5th week on the embryo’s surface.

✅ Correct Answer:

Two placentas, two amniotic cavities, two chorions

Twins that develop from two separate oocytes fertilized by two separate sperm are called dizygotic (fraternal) twins. Each zygote implants independently in the uterus and develops its own placenta, chorion, and amniotic sac.

Thus, dizygotic twins are always dichorionic and diamniotic, though their placentas may sometimes fuse superficially if implantation sites are close together. They share no more genetic similarity than regular siblings and can be of the same or different sexes.

❌ Incorrect Options:

Two placentas, two amniotic cavities, one chorion
A single chorion means a shared outer membrane, which occurs only in monozygotic (identical) twins, not dizygotic.

One placenta, two amniotic cavities, two chorions
Two chorions cannot be enclosed by a single placenta; this combination is anatomically inconsistent.

One placenta, one amniotic cavity, one chorion
This describes monochorionic–monoamniotic twins, which result from late splitting of a single zygote (monozygotic), not from two fertilized oocytes.

None of the above
Incorrect, because dizygotic twins clearly have two placentas, two chorions, and two amniotic cavities.

✅ Correct Answer:

Transitional epithelium

The urinary bladder is lined with transitional epithelium, also known as urothelium. This specialized type of epithelium is designed to stretch and recoil as the bladder fills and empties. In the relaxed state, the cells appear cuboidal to columnar, but when stretched, they become flattened (squamous-like).

The superficial cells, called umbrella cells, form a protective barrier that prevents urine leakage and damage from toxic substances. Transitional epithelium is unique to the urinary tract, lining the renal pelvis, ureters, urinary bladder, and the initial part of the urethra.

❌ Incorrect Options:

Pseudostratified epithelium
This type appears layered but all cells rest on the basement membrane. It lines respiratory passages (like the trachea) and parts of the male reproductive tract, not the bladder.

Simple squamous epithelium
This is a single layer of flat cells found where diffusion or filtration occurs — such as in alveoli and Bowman’s capsule, not in the bladder.

Columnar epithelium
Columnar cells are tall and found in organs like the intestine and gallbladder, where absorption and secretion occur — not suitable for stretch.

Cuboidal epithelium
This type is found in kidney tubules and gland ducts, functioning in secretion and absorption, but lacks the elastic adaptability required in the bladder.

✅ Correct Answer:

Basophilic

Cell structures that stain with basic dyes like hematoxylin are called basophilic. This is because they contain acidic (negatively charged) components such as DNA, RNA, and ribosomes, which attract the positively charged (basic) dye molecules. Thus, the nucleus, nucleolus, and rough endoplasmic reticulum (RER) typically appear blue or purple under the microscope when stained with hematoxylin.

The term basophilic literally means “base-loving,” referring to the affinity of these acidic cellular components for basic dyes.

❌ Incorrect Options:

Eosinophilic
These are structures that stain with acidic dyes like eosin, not basic ones. Eosinophilic (or acidophilic) structures — such as cytoplasmic proteins, mitochondria, and collagen fibers — appear pink or red under the microscope.

Agyrophilic
This term refers to structures that can bind silver salts and become visible after special silver staining techniques (e.g., reticular fibers), not related to basic dyes.

Acidophilic
Synonymous with eosinophilic, acidophilic structures stain with acidic dyes and have a positive charge, opposite of basophilic structures.

None of these
Incorrect, since basophilic is the correct and specific term describing staining with basic dyes.

✅ Correct Answer:

Scanning electron microscope (SEM)

The scanning electron microscope is used for detailed analysis of fibers, hair, soil particles, and other trace evidence in forensic science. SEM provides three-dimensional, high-resolution images by scanning the specimen’s surface with a focused beam of electrons. This allows visualization of surface details at magnifications far beyond light microscopes, making it ideal for identifying fiber morphology, hair cuticle patterns, and microstructures of soil and metal fragments. Additionally, SEM can be coupled with energy-dispersive X-ray spectroscopy (EDS) to determine the elemental composition of materials — a powerful tool in forensic comparison.

❌ Incorrect Options:

Compound microscope
A compound microscope uses visible light and multiple lenses to view thin specimens at moderate magnifications (up to ~1000×). It’s useful for cells and tissues, not high-detail surface analysis of forensic samples.

Simple microscope
This is essentially a single-lens magnifier (like a hand lens). It offers minimal magnification and cannot resolve fine structural details required for forensic comparison.

Phase contrast microscope
Used mainly in biology to observe living, unstained cells by enhancing contrast between transparent structures. It’s unsuitable for non-biological or dry forensic materials.

Bright field microscope
The most common light microscope, using transmitted light to view stained specimens. It lacks the depth and surface detail needed for analyzing tiny fibers or soil morphology.

✅ Correct Answer:

Dorsiflexion

Foot drop refers to the inability to dorsiflex the foot, meaning the person cannot lift the front part of the foot upward at the ankle joint. This occurs when the common peroneal (fibular) nerve or its deep branch is damaged, leading to paralysis or weakness of the tibialis anterior, extensor hallucis longus, and extensor digitorum longus muscles. As a result, the person drags the foot while walking and often compensates with a high-stepping gait to prevent the toes from scraping the ground.

❌ Incorrect Options:

Plantarflexion
This is the movement that points the toes downward (as in pressing a car pedal). It is controlled mainly by the posterior compartment muscles (gastrocnemius, soleus) supplied by the tibial nerve, which is not affected in foot drop.

Circumduction
Circumduction refers to a circular movement involving multiple joints. Although a person with foot drop may appear to swing the leg outward while walking, the movement deficit itself is dorsiflexion, not circumduction.

Inversion
Inversion turns the sole of the foot medially, primarily by tibialis anterior and tibialis posterior. While tibialis anterior is affected, inversion is not completely lost due to tibialis posterior (tibial nerve) remaining functional.

Eversion
Eversion turns the sole laterally and is controlled by the peroneus longus and brevis (superficial peroneal nerve). Foot drop mainly results from deep peroneal nerve injury, so eversion may be mildly affected but is not the defining feature.

✅ Correct Answer:

Intramembranous ossification

Intramembranous ossification is the process in which bone develops directly from mesenchymal tissue, without first forming a cartilage model. This type of ossification is responsible for the formation of flat bones of the skull (such as the frontal and parietal bones), mandible, and clavicle.

In this process, mesenchymal cells differentiate into osteoblasts, which begin secreting osteoid (unmineralized bone matrix). The osteoid later becomes mineralized, forming woven bone, which is then remodeled into lamellar bone. This direct method contrasts with endochondral ossification, which involves a cartilage precursor.

❌ Incorrect Options:

Endochondral ossification
This process involves the formation of a cartilage model first, which is then gradually replaced by bone. It’s typical for long bones such as the femur and humerus.

Perichondral ossification
This refers to bone deposition around the cartilage by the perichondrium, forming a bony collar in developing long bones — part of endochondral ossification, not a separate primary process.

Interosseous ossification
This term is not used to describe any normal developmental process of bone formation. “Interosseous” refers more to the region between bones, such as interosseous membranes, not an ossification pathway.

Intradermal ossification
This term is incorrect; bones do not form within the skin. The confusion may arise because dermal bones (like flat skull bones) arise beneath the skin by intramembranous ossification, not within it.

✅ Correct Answer:

Mitochondria

Mitochondria are unique among organelles because they contain their own circular DNA (mtDNA), separate from the nuclear DNA. This mitochondrial DNA encodes some of the proteins and RNAs required for the mitochondrion’s own function — mainly components of the electron transport chain and ATP synthesis machinery. Mitochondria are often called the “powerhouses of the cell” because they generate ATP through oxidative phosphorylation. Their ability to replicate independently supports the endosymbiotic theory, suggesting they evolved from ancient prokaryotic cells.

❌ Incorrect Options:

Endoplasmic reticulum
The ER is involved in protein and lipid synthesis but does not contain its own DNA. It depends entirely on nuclear DNA for the synthesis of its proteins and enzymes.

Golgi apparatus
This organelle functions as the packaging and modifying center for proteins and lipids but lacks DNA — it relies on enzymes synthesized under nuclear control.

Lysosome
Lysosomes are membrane-bound vesicles containing hydrolytic enzymes for intracellular digestion. They do not have DNA and depend on nuclear instructions for enzyme synthesis.

Ribosome
Ribosomes are the sites of protein synthesis, made of rRNA and proteins, but they do not contain DNA. Instead, they translate mRNA transcribed from nuclear DNA.

✅ Correct Answer:

Sympathetic nervous system

When the students saw the result board and experienced excitement, anxiety, or stress, their sympathetic nervous system was activated — the classic “fight or flight” response. This system prepares the body for action by increasing heart rate, blood pressure, and respiration, dilating pupils, and redirecting blood flow toward skeletal muscles. It helps the body cope with emotional or physical stress by mobilizing energy reserves and sharpening alertness.

❌ Incorrect Options:

Parasympathetic nervous system
This system governs “rest and digest” functions — slowing the heart rate, promoting digestion, and conserving energy. It’s dominant in calm, relaxed states, not during emotional excitement.

Autonomic nervous system
While the sympathetic division is part of the autonomic nervous system (ANS), the ANS as a whole includes both sympathetic and parasympathetic components. The question specifically asks which system was activated, not the broader category.

Enteric nervous system
This controls the gastrointestinal tract, regulating peristalsis and secretions. It operates semi-independently and is unrelated to emotional arousal or excitement.

Somatic nervous system
This system controls voluntary skeletal muscle movement. It’s active when students physically move to the notice board, but the emotional arousal and excitement come from autonomic (sympathetic) activation, not somatic control.

✅ Correct Answer:

Temperature regulation

Eccrine sweat glands play a crucial role in thermoregulation by producing a watery, hypotonic sweat that helps cool the body through evaporative heat loss. These glands are distributed widely across the body, especially on the palms, soles, and forehead, and are controlled by the sympathetic nervous system (cholinergic fibers). When body temperature rises, sweat secretion increases, and as the sweat evaporates from the skin surface, it dissipates heat — maintaining internal temperature homeostasis.

❌ Incorrect Options:

Sebum production
Sebum is an oily secretion produced by sebaceous glands, not eccrine glands. Sebaceous glands are associated with hair follicles and function mainly in lubricating and waterproofing the skin and hair.

Immune response
While sweat has mild antimicrobial properties, the primary immune defense of the skin involves Langerhans cells, keratinocytes, and the skin barrier, not eccrine sweat glands.

Oil secretion
Eccrine glands secrete aqueous (watery) fluid, not oil. Oil secretion is the function of sebaceous glands, which release lipid-rich sebum via holocrine secretion.

Hair growth
Hair growth depends on hair follicles, particularly the matrix cells in the hair bulb, and is influenced by hormones (like androgens) — eccrine sweat glands have no role in this process.