Locomotor – 2021

• Peroneus (fibularis) longus is an evertor of the foot.

• Its tendon passes behind the lateral malleolus, crosses the sole obliquely, and inserts into the:

  • Base of the 1st metatarsal, and

  • Medial cuneiform.

• When paralyzed, inversion predominates (due to tibialis anterior and tibialis posterior), leading to the clinical picture.

• Compression of the tendon at its insertion (medial cuneiform region) confirms its role.

❌ Why the others are wrong

• Cuboid → The tendon of peroneus longus grooves the cuboid bone, but this is only a pulley-like passage; it is not the site of insertion.

• Calcaneus → Insertion site of Achilles tendon (triceps surae), not peroneus longus.

• Lateral cuneiform → Not an insertion site of peroneus longus (though some intrinsic foot muscles attach here).

• Talus → Articulates with tibia and fibula at ankle joint, no peroneus longus insertion.

• Medial and lateral pectoral nerves → purely motor nerves, supplying pectoralis major (both nerves) and pectoralis minor (medial pectoral nerve).

• They do not supply skin → hence, no cutaneous branches.

❌ Why the others are wrong

1. Dorsal scapular nerve originates from the trunk → ❌ Wrong.

   • It actually arises from the root (C5) of the brachial plexus, not from a trunk.

2. The roots divide behind the second part of the axillary artery → ❌ Wrong.

   • Roots form trunks → trunks divide into divisions → cords form around the second part of the axillary artery.

   • It is the cords, not the roots, that are related to the axillary artery.

3. The roots originate behind the scalene muscles → ❌ Wrong.

   • The roots (C5–T1) emerge between the anterior and middle scalene muscles (scalene interval), not behind them.

4. None of these → ❌ Incorrect, since option 5 is a true statement.

• The median cubital vein is a superficial vein in the cubital fossa.

• It lies in the superficial fascia and crosses obliquely over the bicipital aponeurosis (which protects deeper structures like the brachial artery and median nerve).

• It connects the cephalic vein (lateral) with the basilic vein (medial).

• This vein is most commonly used for venipuncture because:

  • It is superficial and easily accessible.

  • It is usually well anchored (less mobile).

  • It has a large diameter.

  • The underlying bicipital aponeurosis protects important deeper structures.

❌ Why the others are wrong

• Median vein → A superficial forearm vein, but not the standard venipuncture site.

• Basilic vein → Runs along medial side of forearm/arm; deeper and less accessible for routine venipuncture.

• Axillary vein → Deep vein in axilla, not used for simple venipuncture.

• Cephalic vein → Runs on the lateral side of forearm/arm; used sometimes for IV access but less commonly for routine blood draw compared to median cubital.

• Type I collagen is the most abundant collagen in the human body (≈90% of total collagen).

• Location: Bone, skin, tendons, ligaments, dentin, fascia, cornea.

• Function: Provides tensile strength and structural support.

❌ Why the others are wrong

• Type II → Found mainly in cartilage, vitreous body, nucleus pulposus. Important for resistance to pressure, but not the most abundant.

• Type III → Found in reticular fibers of skin, blood vessels, granulation tissue. Associated with Ehlers-Danlos syndrome (vascular type).

• Type IV → Found in basement membranes, lens, glomeruli. Forms meshwork, not fibrils.

• Type V → Found in placenta, cornea, some basement membranes, but in very small amounts.

• During contraction, thin (actin) filaments slide over thick (myosin) filaments, pulling the Z-lines closer.

• This changes the length of sarcomere.

• However, the length of the thick filament (myosin) itself does not change, so the A-band remains constant.

❌ Why the others are wrong

• I-Band → shortens (it represents only actin, which slides inward).

• H-Band → shortens and may disappear (it’s the central region of myosin with no actin overlap).

• Z-line → moves closer together, so the distance between them decreases.

• Sarcomere → shortens overall, since contraction pulls Z-lines inward.

• The cords of the brachial plexus (lateral, medial, posterior) are named based on their relationship to the second part of the axillary artery.

• Example:

  • Lateral cord → lateral to axillary artery

  • Medial cord → medial to axillary artery

  • Posterior cord → posterior to axillary artery

Thus, the axillary artery is the key reference point for naming.

❌ Why the others are wrong

• Brachial artery → This is a continuation of the axillary artery below the teres major, but the cords are named higher up, so not the reference.

• Pectoralis minor muscle → The axillary artery itself is divided into three parts based on its relation to pectoralis minor, but the cords are not named in relation to this muscle.

• Pectoralis major muscle → A superficial chest muscle, not related to the naming of cords.

• None of these → Incorrect, since axillary artery is the correct reference.

• Disasters are classified into natural and man-made.

• Bushfires (wildfires) are fires that spread rapidly through vegetation, often triggered by natural causes (lightning, extreme heat, drought) or human activity.

• Since they arise from natural environmental conditions, they are classified under natural disasters.

• They are not purely meteorological (though weather plays a role), nor tectonic (not caused by earth movements), nor biological (not due to organisms), nor topographical (not due to landform instability).

❌ Why the other options are incorrect:

• Tectonic disaster: Caused by earth movements (e.g., earthquakes, volcanic eruptions, tsunamis), not bushfires.

• Meteorological disaster: Caused by weather phenomena (e.g., cyclones, storms, floods); while weather contributes, bushfire is not classified here.

• Topographical hazard: Includes landslides or avalanches related to landform instability, not fire.

• Biological disaster: Involves disease outbreaks, epidemics, or pests — not applicable here.

• In medical ethics, autonomy means that patients have the right to accept or refuse treatment after being fully informed.

• The patient in this case does not believe he has cancer and refuses surgery. The physician cannot force or threaten the patient.

• The best ethical approach is to:

  1. Respect the patient’s decision.

  2. Provide ongoing support and information.

  3. Encourage a second opinion, which might help the patient gain confidence in the diagnosis.

• Weekly follow-ups allow the physician to build trust and keep communication open while respecting autonomy.

❌ Why the other options are incorrect:

• Force him to undergo surgery: Violates patient autonomy and is unethical.

• Detain him and have a meeting with the board: Patients cannot be detained for refusing treatment unless they are a danger to others (not the case here).

• Inform his family and relatives: Violates patient confidentiality unless the patient consents.

• Threaten him if he refuses surgery: Completely unethical and unprofessional.

Dorsiflexion of the foot is performed mainly by the anterior compartment muscles:

• • Tibialis anterior

  • Extensor hallucis longus

  • Extensor digitorum longus

  • Peroneus tertius

• All these muscles are supplied by the deep peroneal (fibular) nerve, a branch of the common peroneal nerve.

• Damage to this nerve leads to loss of dorsiflexion, resulting in foot drop, where the patient cannot lift the toes and must lift the knee high while walking (steppage gait).

❌ Why the other options are incorrect:

• Saphenous nerve: Purely sensory branch of the femoral nerve; supplies sensation to the medial leg, no motor supply.

• Tibial nerve: Supplies posterior compartment of the leg (plantarflexors like gastrocnemius, soleus, tibialis posterior), not dorsiflexors.

• Sural nerve: Purely sensory nerve (posterior leg and lateral foot); no motor function.

• Superficial peroneal nerve: Supplies the lateral compartment muscles (peroneus longus, brevis → eversion of the foot), not dorsiflexors.

The posterior compartment of the thigh contains the hamstring muscles (semimembranosus, semitendinosus, long head of biceps femoris, and the hamstring portion of adductor magnus).

• These muscles primarily originate from the ischial tuberosity, the roughened bony prominence of the ischium.

• From there, they extend across the hip and knee joints, enabling hip extension and knee flexion, which are the hallmark functions of the hamstrings.

❌ Why the other options are incorrect:

• Linea aspera: This is a ridge on the posterior shaft of the femur. While it serves as an attachment for adductor muscles (adductor longus, adductor magnus, etc.), it is not the origin of posterior compartment (hamstring) muscles.

• Medial condyle: This refers to the distal femur and is more relevant for ligament and gastrocnemius attachments, not hamstrings.

• Ischiopubic ramus: This is the origin for some adductor group muscles (like gracilis, adductor brevis), not hamstrings.

• None of these: Incorrect, because we know the hamstrings have a clear, well-defined origin—the ischial tuberosity.

• Limb development begins with small swellings called limb buds, which are derived from mesenchyme covered by ectoderm.

• The upper limb buds appear first at around day 24 of development.

• The lower limb buds appear a few days later, around day 28.

• This slight difference in timing explains why the upper limbs generally develop slightly ahead of the lower limbs.

• This sequence is a highly conserved pattern across vertebrates.

❌ Why the other options are incorrect:

• Limbs of different mammals are different: While limb shapes and functions differ due to adaptations (e.g., wings, flippers, arms), the basic developmental pattern is remarkably conserved across mammals. So this is not the “most accurate” embryological statement.

• Apical ectodermal ridge (AER) has a negative effect: The AER is crucial for limb outgrowth—it secretes FGFs (fibroblast growth factors) that stimulate mesenchymal proliferation. Without it, limb development arrests. So its effect is positive, not negative.

• None of these: Incorrect, since one option (“upper limb buds are evident earlier…”) is clearly correct.

• Limb buds become visible in the sixth week of development: They appear in the fourth week (upper limb ~day 24, lower limb ~day 28), not the sixth.

The intracellular concentration of K+ is significantly higher than the extracellular concentration. Due to this steep concentration gradient, K+ ions have a strong tendency to move out of the cell.

The extracellular concentration of Na+ is much higher than the intracellular concentration. Na+ ions have a strong tendency to move into the cell.

The final resting membrane potential is a compromise between these two opposing forces. The constant outward flow of K+ dominates, setting the potential close to −90 mV, but the small inward leak of Na+ pulls the potential slightly back towards a more positive value. The −86 mV represents this precise balance—the membrane potential is not at the K+ equilibrium potential of −90 mV because of the continuous, though small, influx of Na+ ions.

❌ Why the other options are incorrect:

• Na+-K+ pump: While this pump is absolutely critical for long-term maintenance of the resting potential, it does not cause the −86 mV value. The pump’s role is to restore the ion gradients that are constantly being “run down” by the leaky channels.
• Voltage-gated Na+ channels: These channels are responsible for the rapid depolarization phase of an action potential. They are closed at rest and only open when the membrane potential reaches a specific threshold (e.g., −55 mV).
• Voltage-gated Ca2+ channels: These channels are also not involved in establishing the RMP. They are typically closed at rest and play a role in specific cellular functions, such as neurotransmitter release at the axon terminal or muscle contraction.
• All of these: This is incorrect because, as explained above, voltage-gated channels are closed at the resting potential and therefore do not contribute to it. The primary determinants are the passive leak channels.

The somatic lateral plate mesoderm is a critical component for limb development. It forms the skeletal framework of the limbs, including the scapula, clavicle, humerus, radius, ulna, and the bones of the hands and feet. It also contributes to the connective tissues of the body wall and limbs.

The scapula, being a bone of the pectoral girdle that forms the base of the upper limb, is therefore a direct derivative of the somatic lateral plate mesoderm.

❌ Why the other options are incorrect:

• Neural crest cells:  They contribute to a diverse range of structures, including the peripheral nervous system (e.g., sensory ganglia, Schwann cells), the adrenal medulla, melanocytes (pigment cells), and a significant portion of the craniofacial skeleton (e.g., parts of the jaw, bones of the middle ear). However, they are not responsible for the formation of the appendicular skeleton, which includes the scapula.
• Paraxial mesoderm: This mesoderm forms the somites.
• Splanchnic lateral plate mesoderm: Its primary role is to form the smooth muscle and connective tissue of the gut wall, the serous membranes that cover the organs (like the pleura, peritoneum, and pericardium), and the heart and blood vessels.
• Axial mesoderm: Refers to structures like the notochord, which induces neural tube development and contributes to nucleus pulposus, not to scapular bone.

Skeletal muscle has four major properties:

1. Excitability – ability to respond to a stimulus (usually from a motor neuron).

2. Contractility – ability to shorten and produce force.

3. Extensibility – ability to stretch without being damaged.

4. Elasticity – ability to recoil to its original length after stretching.

❌ Why the other options are incorrect:

• Rhythmicity:
  Seen in cardiac and some smooth muscles (e.g., peristalsis in gut). Skeletal muscle requires nervous input to contract, so it is not rhythmic on its own.

• Syncytium:
  Refers to functional syncytium in cardiac muscle (intercalated discs, gap junctions). Skeletal muscle fibers are multinucleated but not a true syncytium.

• Latch system:
  A property of smooth muscle, where myosin heads remain attached for long periods, maintaining tone with minimal ATP. Not found in skeletal muscle.

• Self-excitation:
  A hallmark of cardiac pacemaker cells (SA node). Skeletal muscle cannot self-excite; it requires stimulation via somatic motor neurons.

When an action potential reaches the muscle fiber membrane (sarcolemma):

• The voltage-gated sodium channels open, allowing Na⁺ ions to rush inside the cell.

• This influx of Na⁺ makes the inside of the muscle fiber less negative (depolarization).

• Once depolarization reaches threshold, the action potential propagates across the sarcolemma and down the T-tubules, leading to excitation-contraction coupling.

So, depolarization in skeletal muscle is primarily caused by Na⁺ influx.

❌ Why the other options are incorrect:

• Ca²⁺:
  Essential for neurotransmitter release at the NMJ and for binding troponin to trigger contraction, but not the main ion for sarcolemma depolarization.

• Cl⁻:
  Chloride influx would make the inside more negative, leading to hyperpolarization, not depolarization.

• K⁺:
  Potassium leaves the cell during repolarization, restoring resting potential, not causing depolarization.

• Mg²⁺:
  Plays a role as an enzymatic cofactor (e.g., ATP-dependent processes) but not in depolarization of muscle membrane.

In medical ethics and professionalism, honesty and transparency with patients are essential.

• A medical student must always introduce themselves accurately as a student, not as a doctor.

• Misrepresenting oneself as a doctor violates ethical principles of truth-telling, informed consent, and professional integrity.

• The teacher disapproved because the student crossed professional boundaries and gave patients the wrong impression about his qualifications.

This is a classic example of an ethical violation, not just a clinical mistake.

❌ Why the other options are incorrect:

• He was proud on his wrong treatment:
  The teacher’s disapproval wasn’t about pride or wrong prescription, but about misrepresentation.

• He did not follow the medical ethic:
  Too broad. The specific ethical breach was introducing himself falsely as a doctor.

• He did not explain the patient’s disease:
  Explanation is important, but the bigger issue is dishonesty in identity, not lack of explanation.

• He discussed it with his teacher:
  Discussing cases with teachers is encouraged, not disapproved.

Compact bone is organized into osteons (Haversian systems), each consisting of a central canal surrounded by concentric lamellae.
The bone matrix is rich in type I collagen, which provides tensile strength and resists pulling forces.
This collagen, along with hydroxyapatite crystals, makes bone both strong and rigid.

❌ Why the other options are incorrect:

• Type VII:
  Found in anchoring fibrils of the basement membrane, not in bone.

• Type IV:
  Found in the basement membrane (forms a meshwork), not bone matrix.

• Type II:
  Found in cartilage, not compact bone.

• Type III:
  Found in reticular fibers (e.g., lymphoid organs, bone marrow), not in compact bone.

Renal osteodystrophy refers to bone disease caused by chronic kidney disease (CKD). CKD → ↓ phosphate excretion → hyperphosphatemia. Phosphate binds calcium, causing ↓ serum calcium → hypocalcemia.
Additionally, diseased kidneys cannot activate vitamin D3 (calcitriol), reducing calcium absorption from the gut.
Hypocalcemia stimulates secondary hyperparathyroidism, which causes increased bone resorption, bone pain, and skeletal deformities.

Thus, the hallmark biochemical features are hypocalcemia + hyperphosphatemia with ↑ parathyroid hormone (PTH).

❌ Why the other options are incorrect:

• Chronic hypophosphatemia and hypocalcemia:
  In CKD, phosphate builds up (hyperphosphatemia), not hypophosphatemia.

• Normal vitamin D3 level:
  Diseased kidneys cannot hydroxylate vitamin D to its active form, so vitamin D3 is low, not normal.

• Normal serum phosphatase level:
  Bone turnover increases alkaline phosphatase (ALP) levels.

• Decreased serum alkaline phosphatase:
  Opposite of what happens. ALP is elevated due to increased osteoblastic activity secondary to PTH-driven bone resorption.

During an action potential, the sequence of events is:

1. Resting state: Membrane at –70 mV, Na⁺ and K⁺ voltage-gated channels closed.

2. Depolarization (rising phase): Voltage-gated Na⁺ channels open, Na⁺ rushes in, membrane potential rises rapidly toward +30 mV.

3. Peak of action potential: As the membrane reaches maximum depolarization, Na⁺ channels begin to inactivate/close. This marks the end of the peak.

4. Repolarization (falling phase): Voltage-gated K⁺ channels open, K⁺ efflux repolarizes the cell.

5. Hyperpolarization: K⁺ channels remain open slightly longer, causing the membrane to dip below resting potential.

6. Restoration: Ion gradients restored via Na⁺/K⁺ pump.

❌ Why the other options are incorrect:

• Opening of Na⁺ channels:
  This causes the start of depolarization, not the end of the peak.

• Closing of Ca²⁺ channels:
  Relevant in cardiac action potentials (plateau phase), not in the typical neuronal action potential.

• Closing of K⁺ channels:
  Happens later, after repolarization, not at the peak.

• Opening of Ca²⁺ channels:
  Again specific to cardiac muscle (plateau phase), not the mechanism for neuronal peak termination.

Compact bone and spongy bone are two structural types of bone tissue:

• Compact bone:

  • Dense, strong, and forms the outer layer of bones.

  • Organized into Haversian systems (osteons) → concentric lamellae around a central Haversian canal containing blood vessels and nerves.

  • Provides mechanical strength.

• Spongy bone (cancellous bone):

  • Consists of trabeculae with bone marrow in between.

  • No Haversian systems.

  • Found mainly in epiphyses of long bones and in flat bones (like cranial bones).

❌ Why the other options are incorrect:

• Thoracic skeleton is made up from this:
  Thoracic skeleton (e.g., sternum, ribs, vertebrae) has both compact and spongy bone. Not unique to compact bone.

• Lines medullary cavity:
  The medullary cavity is lined by spongy bone internally, with compact bone forming the outer wall. Not distinguishing.

• It is also called cancellous bone:
  Cancellous bone is another name for spongy bone, not compact bone.

• Cranial bones are made up from this:
  Cranial bones are primarily spongy (diploë) sandwiched between compact bone layers. Not a unique feature of compact bone.

The surgical neck of the humerus is clinically significant because the axillary nerve and posterior circumflex humeral artery wind around it. A fracture here commonly injures the axillary nerve.

Effects of axillary nerve injury:

• Paralysis of the deltoid and teres minor → loss of abduction of the arm beyond 15°.

• Loss of sensation over the “regimental badge area” on the lateral shoulder.

❌ Why the other options are incorrect:

• Dorsal scapular nerve:
  Innervates rhomboids and levator scapulae. It does not pass near the humerus; hence not injured here.

• Radial nerve:
  Runs in the radial (spiral) groove of the humerus. Injured in midshaft humerus fractures, not at the surgical neck.

• Median nerve:
  Runs anteriorly in the arm, vulnerable at the supracondylar region of the humerus, not the surgical neck.

• Ulnar nerve:
  Passes behind the medial epicondyle of the humerus, injured in fractures of the distal humerus, not the surgical neck.

In smooth muscle, contraction is regulated differently than in skeletal muscle:

1. An increase in intracellular calcium occurs (from extracellular entry or sarcoplasmic reticulum release).

2. Calcium binds to calmodulin, a regulatory protein.

3. The calcium–calmodulin complex activates myosin light chain kinase (MLCK).

4. MLCK phosphorylates the regulatory light chains on myosin heads, allowing them to interact with actin.

5. Cross-bridge cycling occurs → contraction.

Thus, the initiating step is calcium binding to calmodulin.

❌ Why the other options are incorrect:

• Tropomyosin:
  Regulates contraction in skeletal and cardiac muscle by covering actin’s binding sites. In smooth muscle, contraction is not controlled this way.

• Myosin:
  Calcium does not bind directly to myosin. Instead, phosphorylation (via MLCK) regulates its interaction with actin.

• Actin:
  Calcium does not bind to actin in smooth muscle. Actin is the filament myosin interacts with after activation.

• Troponin:
  Absent in smooth muscle. In skeletal and cardiac muscle, calcium binds troponin C to initiate contraction.

The upper part of the femur consists of the head, neck, greater and lesser trochanters, intertrochanteric line and crest, and quadrate tubercle (on the intertrochanteric crest). A fracture in this area can damage these structures.

The adductor tubercle, however, is located distally at the medial epicondyle of the femur, near the knee joint. Therefore, it will not be affected in a fracture of the upper femur.

❌ Why the other options are incorrect:

• Quadrate tubercle:
  Present on the upper femur (on the intertrochanteric crest). Will be affected.

• Head of the femur:
  Clearly part of the upper femur. Can be fractured.

• Neck of the femur:
  The most common fracture site in the elderly. Definitely affected.

• Intertrochanteric line:
  Found on the anterior surface between greater and lesser trochanters, part of the upper femur. Will be affected.

Skeletal muscle contraction begins at the neuromuscular junction (NMJ). The sequence is as follows:

1. Action potential arrives at the axon terminal of the motor neuron.

2. This depolarization opens voltage-gated calcium channels in the presynaptic membrane.

3. Calcium enters the terminal, binding to synaptic vesicles.

4. Vesicles fuse with the presynaptic membrane and release acetylcholine (ACh) into the synaptic cleft.

5. ACh binds to nicotinic receptors on the muscle end plate, causing depolarization and generation of an end plate potential.

6. This leads to propagation of an action potential in the muscle fiber, ultimately triggering contraction.

❌ Why the other options are incorrect:

• Binding of calcium with the neurotransmitter vesicles:
  This happens after the action potential arrives and calcium channels open. Not the first step.

• Excitation of the postsynaptic receptors:
  Occurs only after acetylcholine is released and binds to the motor end plate.

• Opening of the presynaptic calcium channel:
  Triggered by depolarization from the arriving action potential. It’s a subsequent, not initial, event.

• Release of neurotransmitter acetylcholine:
  Also comes later, after calcium influx causes vesicle fusion.

The primary flexors of the elbow joint are:

• Brachialis → the most powerful flexor of the forearm, irrespective of forearm position.

• Biceps brachii → flexes the forearm and also supinates it, most effective when the forearm is supinated.

Other contributors:

• Brachioradialis assists in flexion, especially when the forearm is in mid-pronation (neutral).

• Anconeus is an extensor, not a flexor.

• Coracobrachialis acts at the shoulder joint, not the elbow.

❌ Why the other options are incorrect:

• Anconeus and brachioradialis:
  Anconeus is an extensor, not a flexor. Brachioradialis does help in elbow flexion, but only when the forearm is in neutral; it is not a primary flexor.

• Brachialis and anconeus:
  Brachialis is correct, but anconeus is an extensor, so this pair is wrong.

• None of these:
  Incorrect, since there is a correct option (biceps brachii and brachialis).

• Coracobrachialis and brachialis:
  Coracobrachialis works at the shoulder joint (flexion and adduction), not the elbow. Only brachialis here is correct.

Cell adhesion relies on specialized proteins that mediate interactions between cells or between cells and the extracellular matrix. These proteins maintain tissue architecture, enable signaling, and regulate cell movement. Cadherins, integrins, selectins, and CAMs are all families of adhesion molecules essential for these processes. In contrast, calmodulin is not a cell adhesion protein; it is a calcium-binding regulatory protein inside the cell, important in activating enzymes and signaling pathways, not in adhesion.

❌ Why the other options are incorrect:

• Cadherin:
  A calcium-dependent adhesion molecule responsible for cell-to-cell adhesion in adherens junctions and desmosomes.

• Integrins:
  Mediate cell-to-extracellular matrix adhesion, linking the cytoskeleton to extracellular proteins like fibronectin and laminin.

• Selectins:
  Play a role in transient cell-cell interactions, especially in the immune system (e.g., leukocyte rolling on endothelium).

• Cell adhesion molecules (CAMs):
  A general term for proteins (like NCAM, ICAM, VCAM) that facilitate stable cell-to-cell adhesion and communication.

Osteoclasts are large, multinucleated cells derived from the monocyte–macrophage lineage. Their main function is bone resorption, which requires secretion of enzymes (like collagenases and cathepsins) and acids to dissolve bone matrix. To perform this, osteoclasts have abundant rough endoplasmic reticulum (RER) for protein synthesis and numerous mitochondria for energy. They attach to the bone surface, creating Howship’s lacunae during resorption.

❌ Why the other options are incorrect:

• Housed as isogenous cells in lacunae:
  This describes chondrocytes in cartilage, not osteoclasts. Osteoclasts sit in resorption bays (Howship’s lacunae).

• They communicate through gap junctions:
  This is true for osteocytes, which extend processes through canaliculi and connect via gap junctions. Osteoclasts do not use this mechanism.

• They give rise to osteophytes:
  Osteophytes (bony outgrowths in osteoarthritis) are formed by osteoblast activity, not osteoclasts. Osteoclasts only break down bone.

• They are immature cells:
  Osteoclasts are fully differentiated giant cells. Immature precursors are osteoprogenitor cells (for osteoblasts), not osteoclasts.

The inguinal region, commonly known as the groin, is the area of the body where the abdomen meets the thigh. The superficial inguinal lymph nodes, including the medial group, are situated in this region, lying just below the inguinal ligament.

❌ Why the other options are incorrect:

• Skin of leg: While the lymphatic drainage of the skin of the leg, particularly the medial side, drains into the superficial inguinal lymph nodes, the nodes themselves are not located on the skin of the leg. They are located in the groin.
• Skin of thigh: The superficial inguinal lymph nodes are located in the groin, which is at the very top of the thigh, but not on the general skin of the thigh itself. It’s a more specific location.
• Rectum: While it has lymphatic drainage, the lymph nodes for the lower part of the rectum drain into the inguinal lymph nodes, but the nodes themselves are not found in the rectum. They are located outside the pelvic cavity.
• Penis: The penis’s lymphatic drainage flows to the superficial inguinal lymph nodes. However, the lymph nodes are located in the groin, not within the penis itself.

The palmar aponeurosis is a triangular thickening of deep fascia in the palm. It receives its proximal attachment from the tendon of palmaris longus.
This clinical scenario describes Dupuytren’s contracture, where the palmar aponeurosis thickens and contracts, pulling the fingers (especially 4th and 5th digits) into flexion at the proximal and middle phalanges, limiting extension.

❌ Why the other options are incorrect:

• Flexor digitorum profundus: Inserts into the distal phalanges, responsible for DIP joint flexion, but not the origin of palmar aponeurosis.

• Flexor digitorum superficialis: Inserts into middle phalanges, flexing PIP joints, not connected to palmar aponeurosis.

• Flexor pollicis longus: Flexes the thumb, no relation to palmar aponeurosis.

• Flexor carpi ulnaris: Inserts into pisiform, hook of hamate, and 5th metacarpal base, not the aponeurosis.

The Nernst potential (equilibrium potential) is the diffusion potential that exactly balances and prevents net diffusion of a particular ion across a semipermeable membrane.
It represents the electrical force required to counteract the concentration gradient of that ion.

❌ Why the other options are incorrect:

• Inhibitory postsynaptic potential (IPSP): A graded potential caused by opening of Cl⁻ or K⁺ channels, making the neuron less excitable, not related to ion equilibrium.

• Action potential: A rapid, self-propagating change in membrane potential involving Na⁺ influx and K⁺ efflux, not an equilibrium potential.

• Postsynaptic potential: General term for excitatory or inhibitory changes in membrane potential after neurotransmitter binding, not specific to ion balance.

• Graded potential: Local, variable changes in membrane potential (depolarization or hyperpolarization), not the specific equilibrium point for an ion.

In cases of intermittent locking of the knee, a loose body (often from meniscal tear or osteochondral fragment) may not be visible on routine AP and lateral views.
The tunnel view (intercondylar view) allows visualization of the intercondylar notch and posterior femoral condyles, which are common sites where loose bodies get lodged. This makes it the best additional view in such cases.

❌ Why the other options are incorrect:

• Anteroposterior weight bearing view: Useful for assessing joint space narrowing in arthritis, not loose bodies.

• Skyline view: Visualizes the patellofemoral joint, mainly for patellar tracking and chondromalacia.

• Horizontal beam lateral view: Helpful in trauma cases to detect fractures or effusions, not ideal for loose bodies.

• Oblique view: Used for fractures that may not be clear on standard AP/lateral views, not specifically for intra-articular loose bodies.

The posterior cord of the brachial plexus is formed from the posterior divisions of all three trunks. It gives rise to:

• Axillary nerve → Deltoid, Teres minor

• Radial nerve → Extensors of arm and forearm

• Upper subscapular nerve → Subscapularis

• Thoracodorsal nerve → Latissimus dorsi

• Lower subscapular nerve → Subscapularis and Teres major

Therefore, injury to the posterior cord affects the deltoid (via the axillary nerve), leading to weakness in shoulder abduction.

❌ Why the other options are incorrect:

• Rhomboid major: Supplied by the dorsal scapular nerve (C5 root), not posterior cord.

• Biceps brachii: Supplied by the musculocutaneous nerve (from the lateral cord).

• Pronator teres: Supplied by the median nerve (from lateral and medial cords).

• Subclavius: Supplied by the nerve to subclavius (from the superior trunk).

The cubital fossa is a triangular depression on the anterior aspect of the elbow. Its important contents include:

• Median nerve (most medial of the main structures)

• Brachial artery (just lateral to the median nerve, dividing into radial and ulnar arteries here)

• Biceps brachii tendon (just lateral to the brachial artery)

• Radial nerve (lies more laterally, near the lateral border, under brachioradialis).

❌ Why other options are incorrect:

• All the superficial veins are deep into the cutaneous nerve: Superficial veins (like the median cubital vein) are superficial to fascia, not deep.

• The median nerve is lateral to the brachial artery: The median nerve lies medial to the brachial artery.

• The ulnar nerve is on the medial side: At the elbow, the ulnar nerve passes behind the medial epicondyle, not through the cubital fossa.

• The brachial artery is lateral to the tendon of biceps brachii: The brachial artery is medial to the biceps tendon.

• The patient is elderly with bow legs, bone pain, and progressive hearing loss → classic features of Paget’s disease of bone (osteitis deformans).

• Bone biopsy: shows a mosaic pattern of lamellar bone (diagnostic hallmark).

• Serum calcium, phosphate, and electrolytes are usually normal; only alkaline phosphatase is elevated due to high bone turnover.

• Clinical manifestations include:

  • Bone deformities (bowing, enlargement of skull → hat no longer fits)

  • Neurological symptoms (hearing loss due to compression of cranial nerves, mainly CN VIII)

  • Complications: High-output cardiac failure (due to AV shunts in bone), and rarely osteosarcoma.

❌Why other options are incorrect:

• Giant cell tumor: Occurs in young adults, usually around the knee; shows “soap-bubble” appearance on X-ray, not mosaic bone pattern.

• Chalk stick fracture: Seen in osteopetrosis or advanced osteoporosis, not Paget’s disease.

• High-output cardiac function: Can occur as a complication of Paget’s due to AV shunting, but it’s not the main clinical presentation given here.

• Neurological symptoms: Hearing loss is one neurological symptom, but the hallmark presentation here is bone deformity (bow legs).

Skeletal muscle fibers, unlike highly myelinated axons in the nervous system, are not designed for extremely rapid long-distance conduction. Their primary role is to transmit the signal from the neuromuscular junction across their surface to ensure that the entire fiber contracts simultaneously. The velocity of approximately 2-3 m/s is efficient and sufficient for this purpose. This speed allows for the swift and coordinated release of calcium from the sarcoplasmic reticulum, which is essential for initiating the sliding filament mechanism and muscle contraction.

❌Why the other options are incorrect

• 2-3 mm/s: This value is significantly too slow. A conduction velocity in millimeters per second would mean that the signal would take a very long time to travel even a short distance along the muscle fiber, making rapid and coordinated muscle movements impossible.
• 20-30 m/s: This value is too fast for a skeletal muscle fiber. This range of speed is more characteristic of highly myelinated, large-diameter nerve fibers (like Aα motor neurons) that transmit signals from the central nervous system to the muscle.
• 3-4 m/s: While close, this is generally considered a slight overestimation of the typical average velocity.
• 1-2 m/s: This value is a bit on the lower end of the accepted range.

Teres major is supplied by the lower subscapular nerve (C5, C6), a branch of the posterior cord of the brachial plexus.

Functionally, teres major helps in medial rotation, extension, and adduction of the humerus.

❌ Why the other options are incorrect:

• Radial nerve: Supplies the extensor muscles of the arm and forearm, not teres major.

• Axillary nerve: Innervates deltoid and teres minor, not teres major.

• Upper subscapular nerve: Innervates subscapularis, not teres major.

• Suprascapular nerve: Innervates supraspinatus and infraspinatus, not teres major.

Complete tetanus refers to a state of continuous, sustained contraction with no relaxation between successive stimuli. This happens when the frequency of stimulation is so rapid that the muscle does not have time to enter relaxation, resulting in a smooth, sustained contraction.

❌ Why the other options are incorrect:

• Incomplete tetanus: Here, partial relaxation is still present between contractions, so it is not continuous.

• Treppe (staircase phenomenon): Muscle contractions gradually increase in strength with repeated stimulation, but relaxation still occurs.

• Wave summation: This is the additive effect of successive stimuli before complete relaxation, but still not continuous contraction.

• Twitch: A single, quick contraction followed by relaxation — the opposite of sustained contraction.

Hurler syndrome (MPS I) is caused by a deficiency of alpha-L-iduronidase, a lysosomal enzyme needed for the degradation of dermatan sulfate and heparan sulfate. Without this enzyme, these glycosaminoglycans accumulate in tissues, leading to developmental delay, coarse facial features, hepatosplenomegaly, corneal clouding, and skeletal abnormalities.

❌ Why the other options are incorrect:

• Iduronate-2-sulfatase: Deficient in Hunter syndrome (MPS II).

• Heparin sulfamidase: Deficient in Sanfilippo syndrome (MPS IIIA).

• N-acetyl-glucosaminidase: Deficient in Sanfilippo syndrome (MPS IIIB).

• Beta-glucuronidase: Deficient in Sly syndrome (MPS VII).

During collagen synthesis, hydroxylation of proline and lysine residues is essential for the stability of the collagen triple helix. This reaction requires ascorbic acid (Vitamin C) as a cofactor for the enzymes prolyl hydroxylase and lysyl hydroxylase.
In the absence of Vitamin C, defective hydroxylation leads to weak collagen, resulting in scurvy (bleeding gums, poor wound healing, fragile blood vessels).

❌ Why the other options are incorrect:

• Vitamin B: General term; not specifically involved in hydroxylation of collagen.

• Vitamin E: Functions mainly as an antioxidant; not required for collagen synthesis.

• Vitamin B12: Important for DNA synthesis and red blood cell maturation; no role in collagen hydroxylation.

• Vitamin A: Involved in vision, epithelial differentiation, and bone growth; not directly linked to hydroxylation in collagen synthesis.

The gluteus maximus is the strongest extensor of the hip joint. It is especially important during activities requiring forceful hip extension, such as climbing stairs, running, or rising from a seated position. Weakness in this muscle leads to difficulty with such movements.

❌ Why the other options are incorrect:

• Piriformis: A lateral rotator of the hip; does not play a primary role in stair climbing.

• Gluteus medius: Abductor of the hip and stabilizer of the pelvis during walking; weakness causes a Trendelenburg gait, not stair-climbing difficulty.

• Biceps femoris: Flexes the knee and extends the hip, but it is not the main extensor for powerful actions like climbing stairs.

• Obturator externus: A lateral rotator of the thigh; not significant for stair climbing.

The Fenn effect states that the amount of work done by skeletal muscle is directly proportional to the amount of ATP hydrolyzed. In other words, when muscle does more work (shortening against a load), it consumes more ATP. This phenomenon shows the tight coupling between chemical energy (ATP hydrolysis) and mechanical energy (contraction) in muscle physiology.

❌ Why the other options are incorrect:

• Work done is inversely proportional to ATP cleaved: Opposite of the Fenn effect; more ATP hydrolysis equals more work, not less.

• Work done is directly proportional to ATP and Mg²⁺ cleaved: Mg²⁺ is a cofactor for ATPase activity but is not cleaved, so this is inaccurate.

• Work done is inversely proportional to ATP and Mg²⁺ cleaved: Same error as above, and also the relationship is direct, not inverse.

• Work done is directly proportional to Ca²⁺ cleaved:  Calcium is not “cleaved”; it regulates contraction by binding troponin, not by being broken down for energy.

Cartilage is primarily composed of Type II collagen fibers, which are thin fibrils designed to resist compressive stress and provide tensile strength in structures like hyaline and elastic cartilage. They are associated with proteoglycans (e.g., aggrecan) that allow cartilage to withstand pressure.

❌ Why the other options are incorrect:

• Type 4: Found in the basement membrane (forms a network, not fibrils).

• Type 1: Major collagen of bone, tendons, ligaments, skin, providing tensile strength, not typical of cartilage.

• Type 6: Found in the intervertebral disc and connective tissue interfaces, not the primary collagen in cartilage.

• Type 3: Known as reticular fibers, present in lymphoid tissues and bone marrow, not in cartilage.

Methylxanthines (like theophylline, aminophylline) act mainly by inhibiting the enzyme phosphodiesterase (PDE). Normally, PDE breaks down cyclic AMP (cAMP). By inhibiting PDE, methylxanthines increase intracellular cAMP, which leads to:

• Bronchodilation (relaxation of smooth muscle)

• Anti-inflammatory effects (reduced mediator release)
  Additionally, methylxanthines block adenosine receptors, contributing to bronchodilation and CNS stimulation.

❌ Why the other options are incorrect:

• Beta-adrenergic receptor stimulation: This is the mechanism of β₂ agonists (e.g., salbutamol), not methylxanthines.

• Alpha-adrenergic receptor stimulation: This causes vasoconstriction (seen in drugs like phenylephrine), unrelated to methylxanthines.

• Regulation of intracellular fluoride levels: Not a physiological mechanism in pharmacology.

• Regulation of intracellular sodium levels: This is related to sodium channels or sodium-potassium pump activity, not the action of methylxanthines.

The great saphenous vein (GSV) is the longest vein in the body, running from the medial side of the foot up to the groin, where it joins the femoral vein at the saphenofemoral junction.

• Importantly, the GSV runs superficial to the fascia lata (deep fascia of the thigh), which makes it easily accessible for surgical harvesting in procedures like coronary artery bypass grafting (CABG).

❌ Why the other options are incorrect:

• It lies anterior to the medial condyle of the femur: The vein passes anterior to the medial malleolus (ankle), not the femoral condyle.

• It lies posterior to medial malleolus: The GSV runs anterior to the medial malleolus; the small saphenous vein is posterior.

• It passes deep to the fascia lata of the thigh:  Its surgical usefulness comes from being superficial, not deep.

• It originates from the femoral vein: It drains into the femoral vein, not originates from it.

The iliopsoas is the primary hip flexor. If it is paralyzed, other muscles that can contribute to hip flexion may undergo hypertrophy to compensate. Among these:

• Rectus femoris is part of the quadriceps group. It is biarticular, crossing both the hip and knee joints. Its proximal attachment at the anterior inferior iliac spine allows it to assist in hip flexion when iliopsoas is paralyzed.

• Other hip flexors like pectineus and tensor fascia latae assist, but rectus femoris contributes more significantly under load, making it the main compensatory muscle.

❌ Why the other options are incorrect:

• Pectineus: Small adductor and weak hip flexor; less likely to hypertrophy enough for major compensation.

• Vastus intermedius: Crosses only the knee joint; does not assist in hip flexion.

• Tensor fascia latae: Assists in hip flexion and abduction, but its contribution is minor compared to rectus femoris.

• Semimembranosus: Part of hamstrings; primarily hip extension and knee flexion, opposite action to hip flexion.

Cyclones are intense weather systems characterized by strong winds, heavy rain, and low pressure. They are classified as meteorological disasters because:

• They originate in the atmosphere, over oceans or seas.

• They are natural disasters caused by weather phenomena, not by earthquakes, human activity, or radioactive events.

• Their effects include flooding, property damage, and human casualties.

❌ Why the other options are incorrect:

• Telluric and tectonic: Refers to earthquakes and volcanoes, disasters originating from the Earth’s crust.

• Accident: Refers to human-made mishaps like industrial or transportation accidents.

• Atomic explosion: Refers to nuclear disasters, caused by radiation release.

• Topological: Not a standard disaster classification; might relate to terrain but not applicable to cyclones.

Claw hand occurs due to paralysis of the intrinsic muscles of the hand (mainly lumbricals and interossei) that flex the metacarpophalangeal (MCP) joints and extend the interphalangeal (IP) joints.

• The ulnar nerve supplies:

  • All interossei (palmar and dorsal)

  • Medial two lumbricals

  • Hypothenar muscles

• Damage to the ulnar nerve (commonly at the elbow or wrist) leads to hyperextension at MCP joints and flexion at IP joints of the 4th and 5th digits, giving the characteristic “claw” appearance.

❌ Why the other options are incorrect:

• Radial nerve: Injury causes wrist drop, not claw hand.

• Median nerve: Injury (at wrist or elbow) causes “ape hand” or thenar wasting, affecting thumb opposition, not clawing of 4th/5th fingers.

• Axillary nerve: Innervates deltoid and teres minor; injury causes loss of shoulder abduction, not claw hand.

• Musculocutaneous nerve: Supplies anterior arm muscles; injury leads to weak elbow flexion but no hand deformity.

The popliteus muscle is a small muscle located at the posterior aspect of the knee. Its main function is to “unlock” the knee from full extension by:

• Medially rotating the tibia on the femur (or laterally rotating the femur on the tibia in a fixed leg)

• This initiates knee flexion, allowing other muscles like the hamstrings to continue the movement.

Other muscles contribute to knee movement but do not perform the unlocking action:

❌ Why the other options are incorrect:

• Quadriceps femoris: Extends the knee; cannot unlock it.

• Plantaris: Weak flexor of knee and ankle; does not initiate unlocking.

• Biceps femoris: Flexes the knee, but after the knee is already unlocked.

• Gastrocnemius: Assists in knee flexion, but does not rotate or unlock the joint.

Lambert-Eaton Myasthenic Syndrome (LEMS) is an autoimmune disorder affecting the neuromuscular junction. In LEMS:

• Autoantibodies are produced against voltage-gated calcium channels (VGCCs) on the presynaptic nerve terminal.

• Blocking these channels reduces calcium influx, which is necessary for acetylcholine release.

• This leads to muscle weakness, especially in proximal muscles, and autonomic symptoms like dry mouth.

❌ Why the other options are incorrect:

• Sodium channels: Autoantibodies to these are involved in certain channelopathies, not LEMS.

• Potassium channels: Not targeted in LEMS; involved in repolarization of nerve/muscle membranes.

• Acetylcholine channels (nicotinic receptors): Targeted in myasthenia gravis, not LEMS.

• Muscle proteins: Autoantibodies against structural muscle proteins occur in inflammatory myopathies, not LEMS.

The Nernst equation relates the equilibrium potential (diffusion potential) of an ion to its concentration difference across a semipermeable membrane. It predicts the voltage at which there is no net movement of that particular ion because the electrochemical driving force is balanced.

It considers one ion at a time, not multiple ions simultaneously.

It applies to ions that are permeable through the membrane.

❌ Why the other options are incorrect:

• Counter transport: Refers to coupled movement of two ions in opposite directions; Nernst equation does not account for this.

• Concentration of multiple ions: That is described by the Goldman-Hodgkin-Katz equation, not Nernst.

• Facilitated diffusion: Mechanism of transport; Nernst describes equilibrium potential, not transport kinetics.

• Co-transport: Movement of two ions in the same direction; not what the Nernst equation addresses.

Chondroitin sulfate is a glycosaminoglycan (GAG) found in cartilage, tendons, ligaments, and aorta. Its repeating disaccharide unit is composed of:

• N-acetylgalactosamine (amino sugar)

• Glucuronic acid (uronic acid)

These disaccharides are sulfated at various positions, giving chondroitin sulfate its negative charge and ability to attract water, which provides compressive strength and elasticity to cartilage.

❌ Why the other options are incorrect:

• N-acetylglucosamine + iduronic acid: Forms part of heparan sulfate, not chondroitin sulfate.

• Glucuronic acid + iduronic acid: Found in dermatan sulfate, but lacks an amino sugar.

• N-acetylglucosamine + glucuronic acid: Forms part of hyaluronic acid, not sulfated.

• N-acetylgalactosamine + iduronic acid: Found in dermatan sulfate, not chondroitin sulfate.

The common peroneal nerve (common fibular nerve) is a branch of the sciatic nerve. It winds around the neck of the fibula just below the lateral knee, making it highly susceptible to injury from:

• Direct trauma (blows to the lateral knee)

• Fibular neck fractures

• Tight casts or prolonged leg crossing

Injury to the common peroneal nerve can lead to foot drop (loss of dorsiflexion) and sensory deficits on the lateral leg and dorsum of the foot.

❌ Why the other options are incorrect:

• Superficial fibular nerve: A branch of the common peroneal nerve; injury is secondary and occurs more distally.

• Tibial nerve: Lies posteriorly in the leg; not vulnerable to fibular neck fractures.

• Deep fibular nerve: Also a branch of the common peroneal nerve; injury is typically distal near the anterior compartment.

• Sciatic nerve: Located proximally in the thigh; injury would require more proximal trauma, not isolated fibular neck fracture.

Winging of the scapula occurs when the medial border of the scapula protrudes posteriorly, especially when pushing against a wall. This happens due to paralysis of the serratus anterior muscle, which normally holds the scapula flat against the rib cage.

The long thoracic nerve innervates the serratus anterior. Injury to this nerve (commonly due to trauma or after surgery, like mastectomy) prevents the serratus anterior from functioning, resulting in scapular winging.

❌ Why the other options are incorrect:

• Nerve to subclavius: Innervates the subclavius muscle; injury does not cause scapular winging.

• Lateral pectoral nerve: Supplies pectoralis major; injury affects shoulder flexion/adduction but does not produce winging.

• Dorsal scapular nerve: Supplies rhomboids; injury may impair scapular retraction but does not cause the classic winging.

• Medial pectoral nerve: Supplies pectoralis minor (and part of major); injury does not result in scapular winging.

The medial longitudinal arch of the foot is composed of the calcaneus, talus, navicular, three cuneiforms, and the first three metatarsals. The keystone is the talus head, located at the apex of the arch, because:

• It transmits body weight from the tibia to the rest of the foot.

• Its position allows the arch to flexibly adjust to loads while maintaining the shape of the foot.

• The surrounding bones and ligaments support the talus, but it bears the primary weight and maintains the arch’s integrity.

❌ Why the other options are incorrect:

• Navicular bone: Supports the arch but acts more as a supporting structure, not the central keystone.

• Calcaneal tubercle: Forms the posterior base of the arch; important for weight-bearing, but not the keystone.

• Medial cuneiform bone: Contributes to the arch structure, but not centrally located as the keystone.

• Metatarsals: Form the distal part of the arch, transmitting forces to the toes; not the keystone.

Howship’s lacunae are small depressions or cavities on the bone surface where bone resorption occurs. These lacunae are formed by the osteoclasts, which are large, multinucleated cells that break down bone matrix by secreting acids and proteolytic enzymes. This process is essential for bone remodeling and calcium homeostasis.

❌ Why the other options are incorrect:

• Osteoblasts: Bone-forming cells; they synthesize new bone rather than resorb it, and are not found in Howship’s lacunae.

• Osteocytes: Mature bone cells embedded in the lacunae of the bone matrix, not on the surface resorption pits.

• Chondrocytes: Cartilage cells; unrelated to bone resorption or Howship’s lacunae.

• Osteoprogenitor cells: Precursors of osteoblasts; located in the periosteum and endosteum, not in resorption lacunae.

The sartorius is the longest muscle in the human body. It originates from the anterior superior iliac spine (ASIS) and inserts onto the medial surface of the tibia (part of the pes anserinus). It acts to flex, abduct, and laterally rotate the hip, and flex the knee, allowing movements like crossing the legs.

❌ Why the other options are incorrect:

• Pectineus: Short adductor/flexor of the hip; not the longest.

• Iliacus: Fan-shaped and relatively short; not spanning the thigh length.

• Psoas major: Long, but primarily vertical from lumbar spine to femur; does not cross the knee.

• Vastus medialis: Quadriceps muscle of the thigh; does not extend from hip to tibia.

Pyogenic osteomyelitis is a bacterial infection of bone that produces pus formation. The most common causative organism is Staphylococcus aureus, which:

• Has surface proteins that allow it to adhere to bone matrix and prosthetic material.

• Produces toxins and enzymes that promote bone destruction and abscess formation.

• Can infect both children and adults, though the sites differ (long bones in children, vertebrae in adults).

Other organisms can cause osteomyelitis but are less common and often associated with special circumstances.

❌ Why the other options are incorrect:

• Salmonella typhi: Causes osteomyelitis mainly in sickle cell disease, not the general population.

• Staphylococcus epidermidis: Usually associated with prosthetic implants, not native bone infection.

• Pseudomonas aeruginosa: Can cause osteomyelitis in IV drug users or puncture wounds, rare otherwise.

• Mycobacterium tuberculosis: Causes tuberculous osteomyelitis (Pott’s disease), which is chronic and not pyogenic.

Neuronal action potentials are generated when the membrane potential reaches threshold. Certain ions can inhibit or prevent depolarization.

Chloride ions (Cl⁻) are negatively charged. When they enter the neuron through GABA_A or glycine receptor channels, they hyperpolarize the membrane, moving it further from the threshold and inhibiting action potential generation.

Other ions have different effects.

❌ Why the other options are incorrect:

• Influx of potassium: Normally potassium exits the cell to repolarize; inward movement is rare and does not inhibit AP effectively.

• Influx of bicarbonate: Minor pH effects, not a direct inhibitor of AP.

• Influx of calcium: Contributes to depolarization and synaptic activity.

• Influx of sodium: Directly triggers depolarization, not inhibition.