General Physiology — MCQs

General Physiology Indian Medical PG Practice Questions and MCQs

Question 101: Na+-K+ ATPase is which type of pump?

A. Secondary active
B. Electrogenic (Correct Answer)
C. Symport
D. All of the above

Explanation: ### Explanation The **Na⁺-K⁺ ATPase** (Sodium-Potassium Pump) is a primary active transporter found in the plasma membrane of almost all animal cells. **Why "Electrogenic" is the Correct Answer:** The pump moves **3 Na⁺ ions out** of the cell and **2 K⁺ ions into** the cell for every molecule of ATP hydrolyzed. Because there is an unequal exchange of cations (3 positive charges out vs. 2 positive charges in), it creates a net deficit of positive charge inside the cell. This directly contributes to the negativity of the **Resting Membrane Potential (RMP)**, typically contributing about -5 to -10 mV. Any pump that creates a net charge imbalance across the membrane is termed **electrogenic**. **Analysis of Incorrect Options:** * **A. Secondary active:** The Na⁺-K⁺ ATPase is a **Primary Active Transporter** because it derives energy directly from the hydrolysis of ATP (it is a P-type ATPase). Secondary active transport (like SGLT) relies on the gradient created by primary active transport. * **C. Symport:** This pump is an **Antiport** (counter-transport) mechanism because it moves the two ions in opposite directions across the cell membrane. * **D. All of the above:** Incorrect, as options A and C are physiologically inaccurate descriptions of this specific pump. **High-Yield Clinical Pearls for NEET-PG:** * **Inhibitor:** The pump is specifically inhibited by **Cardiac Glycosides** (e.g., Digoxin and Ouabain), which bind to the extracellular side of the alpha subunit. * **Structure:** It is a heteromer composed of an **alpha subunit** (catalytic, contains binding sites for Na⁺, K⁺, and ATP) and a **beta subunit** (essential for membrane trafficking). * **Functions:** It maintains cell volume, provides the driving force for secondary active transport, and is crucial for maintaining the steep Na⁺ and K⁺ gradients necessary for excitable tissues.

Question 102: Which of the following is NOT a major calcium-binding protein?

A. Troponin
B. Calmodulin
C. Actinin (Correct Answer)
D. Calcineurin

Explanation: **Explanation:** The correct answer is **Actinin (Option C)**. **Why Actinin is the correct answer:** Actinin (specifically $\alpha$-actinin) is a **structural protein**, not a calcium-binding protein. Its primary role is to anchor actin filaments to the Z-discs in skeletal muscle and to dense bodies in smooth muscle. It belongs to the spectrin superfamily and functions as a cross-linking protein to maintain the architectural integrity of the sarcomere. **Analysis of Incorrect Options:** * **Troponin (Option A):** Specifically, **Troponin C** is a major calcium-binding protein in striated muscle. Upon binding $Ca^{2+}$, it undergoes a conformational change that moves tropomyosin away from the myosin-binding sites on actin, initiating contraction. * **Calmodulin (Option B):** This is the most ubiquitous calcium-binding messenger protein. It mediates various cellular processes (like activating Myosin Light Chain Kinase in smooth muscle) by forming a $Ca^{2+}$-calmodulin complex. * **Calcineurin (Option D):** Also known as Protein Phosphatase 2B, it is a calcium and calmodulin-dependent serine/threonine protein phosphatase. It plays a critical role in T-cell activation and cardiac hypertrophy. **High-Yield Clinical Pearls for NEET-PG:** * **Calsequestrin:** The major $Ca^{2+}$-binding protein located inside the **Sarcoplasmic Reticulum** (SR) that allows for high-capacity $Ca^{2+}$ storage. * **Calbindin:** A vitamin D-dependent calcium-binding protein found in the intestines and kidneys that facilitates $Ca^{2+}$ absorption/reabsorption. * **Clinical Correlation:** Calcineurin inhibitors (e.g., **Cyclosporine, Tacrolimus**) are potent immunosuppressants used to prevent organ transplant rejection.

Question 103: Saltatory conduction in myelinated axons results from the fact that:

A. Salt concentration is increased beneath the myelin segments
B. Non-gated ion channels are present beneath the segments of myelin
C. Membrane resistance is decreased beneath the segments of myelin
D. Voltage-gated sodium channels are concentrated at the nodes of Ranvier (Correct Answer)

Explanation: **Explanation:** **Why the correct answer is right:** Saltatory conduction (from the Latin *saltare*, meaning "to leap") is the rapid propagation of action potentials along myelinated axons. Myelin acts as an electrical insulator, significantly increasing membrane resistance and decreasing membrane capacitance. Because of this insulation, the ionic current cannot flow through the membrane in the myelinated segments. Instead, the action potential "jumps" from one **Node of Ranvier** to the next. This is possible because **Voltage-gated Sodium (Na+) channels** are highly concentrated at these nodes (approximately 2000–12000 per $\mu m^2$), while they are virtually absent in the internodal segments beneath the myelin. This concentration allows for rapid depolarization only at the nodes, conserving energy and increasing conduction velocity. **Why the incorrect options are wrong:** * **Option A:** Saltatory conduction is related to electrical insulation and channel distribution, not the concentration of physical salts (NaCl) beneath the myelin. * **Option B:** If non-gated (leak) channels were the primary feature beneath myelin, the current would leak out, dissipating the signal rather than conducting it rapidly. * **Option C:** Myelin actually **increases** membrane resistance ($R_m$). Decreased resistance would mean the membrane is "leaky," which would slow down or stop the conduction of the impulse. * **Note on Capacitance:** Myelin **decreases** membrane capacitance ($C_m$), which allows the membrane potential to change more rapidly with less charge displacement. **High-Yield Clinical Pearls for NEET-PG:** * **Conduction Velocity:** Proportional to the diameter in myelinated fibers ($V \propto \text{diameter}$) and proportional to the square root of the diameter in unmyelinated fibers ($V \propto \sqrt{\text{diameter}}$). * **Demyelinating Diseases:** In conditions like **Multiple Sclerosis** (CNS) or **Guillain-Barré Syndrome** (PNS), the loss of myelin increases capacitance and decreases resistance, leading to "conduction block" or slowing of the impulse. * **Energy Efficiency:** Saltatory conduction is energy-efficient because the $Na^+-K^+$ ATPase pump only needs to work at the nodes to restore ionic gradients.

Question 104: What is the term for the transport of sodium and potassium in opposite directions?

A. Symport
B. Na-K ATPase
C. Neutral transport
D. Antiport (Correct Answer)

Explanation: **Explanation:** The correct answer is **Antiport** (also known as counter-transport). This is a type of secondary active transport (or mediated transport) where two different molecules or ions are moved across a cell membrane in **opposite directions** simultaneously. In the context of sodium and potassium, an example is the Na⁺-H⁺ exchanger or specific Na⁺-K⁺ exchangers found in certain tissues, where one ion moves down its electrochemical gradient to provide the energy for the other to move against its gradient. **Analysis of Options:** * **A. Symport:** Also known as co-transport, this involves the movement of two substances in the **same direction** (e.g., SGLT-1 moving Sodium and Glucose together into the intestinal cell). * **B. Na-K ATPase:** While this pump does move Na⁺ and K⁺ in opposite directions (3 Na⁺ out, 2 K⁺ in), it is a specific **primary active transport mechanism** (a protein pump), not the general term for the directional classification of transport. * **C. Neutral transport:** This is a non-specific term. In physiology, it usually refers to the movement of uncharged molecules or a transport process that does not result in a net change in charge across the membrane (electroneutral). **High-Yield Clinical Pearls for NEET-PG:** * **Primary vs. Secondary:** Na-K ATPase is **Primary** active transport (uses ATP directly). Antiport/Symport are usually **Secondary** active transport (use the gradient created by primary transport). * **Electrogenicity:** The Na-K ATPase is **electrogenic** because it pumps 3 positive charges out for every 2 it brings in, contributing to the resting membrane potential. * **Inhibitor:** Digitalis (Digoxin) inhibits the Na-K ATPase, leading to increased intracellular Na⁺, which subsequently slows the Na⁺-Ca²⁺ exchanger (an antiport), increasing intracellular Calcium and cardiac contractility.

Question 105: What is the approximate osmolarity of human plasma?

A. 190 mOsm/L
B. 290 mOsm/L (Correct Answer)
C. 390 mOsm/L
D. 490 mOsm/L

Explanation: **Explanation:** The osmolarity of human plasma is a measure of the concentration of osmotically active particles in a liter of solution. In a healthy adult, the normal range for plasma osmolarity is approximately **280 to 295 mOsm/L**, making **290 mOsm/L** the most accurate representative value among the choices. **Why the correct answer is right:** Plasma osmolarity is primarily determined by electrolytes, specifically **Sodium (Na⁺)** and its associated anions (Chloride and Bicarbonate), which account for nearly 90% of the total osmotic pressure. Glucose and Urea contribute the remaining fraction. The body tightly regulates this value via the hypothalamus-pituitary-adrenal axis (ADH secretion) and the thirst mechanism to maintain cellular volume and homeostasis. **Why the incorrect options are wrong:** * **A (190 mOsm/L):** This value represents extreme hypotonicity. Such low osmolarity would cause massive water shift into cells (cerebral edema), leading to seizures or death. * **C & D (390 & 490 mOsm/L):** These values represent severe hypertonicity. High osmolarity (seen in conditions like Hyperosmolar Hyperglycemic State or severe dehydration) causes cellular shrinkage and neurological dysfunction. **High-Yield NEET-PG Pearls:** 1. **Calculated Osmolarity Formula:** $2 \times [Na^+] + \frac{[Glucose]}{18} + \frac{[BUN]}{2.8}$. 2. **Osmolar Gap:** The difference between measured and calculated osmolarity. A gap **>10 mOsm/L** suggests the presence of unmeasured osmoles (e.g., Ethanol, Methanol, Ethylene glycol). 3. **Osmolality vs. Osmolarity:** While often used interchangeably in clinical practice, *Osmolality* (mOsm/kg) is more accurate as it is independent of temperature and pressure. Normal plasma osmolality is similarly ~285–295 mOsm/kg.

Question 106: The Lewis Triple Response is mediated by which of the following?

A. Histamine (Correct Answer)
B. Axon reflex
C. Injury to endothelium
D. None of the above

Explanation: **Explanation:** The **Lewis Triple Response** is a characteristic cutaneous reaction that occurs following firm stroking of the skin or intradermal injection of certain substances. It is primarily mediated by the release of **Histamine** from mast cells. **1. Why Histamine is the Correct Answer:** Histamine is the central chemical mediator of this response. When the skin is injured, mast cells degranulate, releasing histamine which acts on H1 receptors to produce three distinct stages: * **Red Reaction (Flush):** Localized capillary dilatation (occurs within seconds). * **Flare:** A spreading redness beyond the site of injury caused by arteriolar dilatation. This is mediated by the **axon reflex**, but histamine is the trigger that initiates this neural pathway. * **Wheal:** Localized edema due to increased capillary permeability (exudation of fluid). **2. Analysis of Incorrect Options:** * **Option B (Axon Reflex):** While the "Flare" component is specifically mediated by the axon reflex (antidromic conduction), the axon reflex itself is a *mechanism* triggered by histamine. Histamine is the primary *mediator* for the entire triple response. * **Option C (Injury to endothelium):** While endothelial changes occur (contraction of endothelial cells leading to gaps), the response is a physiological reaction to chemical mediators rather than a direct mechanical injury to the endothelium itself. **High-Yield Clinical Pearls for NEET-PG:** * **Dermatographism:** An exaggerated triple response where even light pressure causes a wheal; it is a form of physical urticaria. * **Antidromic Conduction:** The flare is unique because the impulse travels "backwards" along sensory nerve fibers without involving the spinal cord. * **Substance P:** This neuropeptide is often released alongside histamine during the axon reflex to further promote vasodilation.

Question 107: What is the daily insensible water loss through the respiratory tract?

A. 100 ml
B. 200 ml
C. 350 ml (Correct Answer)
D. 700 ml

Explanation: **Explanation:** Insensible water loss refers to the continuous, unavoidable loss of water that occurs without the individual's awareness, primarily through the skin (diffusion) and the respiratory tract (evaporation). **1. Why 350 ml is correct:** In a healthy adult under normal conditions, the total daily insensible water loss is approximately **700 ml**. This is divided almost equally between the skin and the lungs. As air enters the respiratory tract, it is humidified to a vapor pressure of about 47 mmHg before reaching the alveoli. This process of humidification results in a daily loss of approximately **300–400 ml** (average **350 ml**) of water through exhalation. **2. Analysis of Incorrect Options:** * **A (100 ml):** This value is too low for respiratory loss. However, 100 ml is the typical daily volume of water lost in **feces** under normal conditions. * **B (200 ml):** This is an underestimate for respiratory loss but may represent sweat loss in a sedentary individual in a temperate climate. * **D (700 ml):** This represents the **total** daily insensible water loss (Skin + Lungs). The question specifically asks for the respiratory component only. **High-Yield Clinical Pearls for NEET-PG:** * **Effect of Temperature:** In cold weather, atmospheric vapor pressure decreases to nearly zero, increasing the gradient for evaporation; this explains the "foggy breath" and increased respiratory water loss in winter. * **Clinical Significance:** Insensible loss is **solute-free water**. In patients with extensive burns, the "insensible" loss through the skin can increase from 350 ml to 3–5 Liters/day due to the loss of the cornified layer. * **Fever:** For every 1°C rise in body temperature, insensible water loss increases by approximately 10–15%.

Question 108: Along with acetylcholine, what is released from sweat gland secretions?

A. Neuro peptide Y
B. Enkephalin
C. Vasoactive intestinal peptide (Correct Answer)
D. Substance P

Explanation: **Explanation:** The correct answer is **Vasoactive intestinal peptide (VIP)**. **1. Why VIP is correct:** Sweat glands are unique in the autonomic nervous system. While they are anatomically part of the **sympathetic nervous system**, their postganglionic fibers are **cholinergic** (releasing Acetylcholine). In these specific fibers, **Vasoactive Intestinal Peptide (VIP)** is co-released with Acetylcholine (ACh). * **Mechanism:** ACh acts on muscarinic receptors to stimulate sweat secretion. VIP acts as a potent vasodilator, increasing local blood flow to the sweat glands, which provides the necessary fluid for sweat production and aids in thermoregulation by facilitating heat loss. **2. Why other options are incorrect:** * **A. Neuropeptide Y (NPY):** This is typically co-released with **Norepinephrine** in sympathetic adrenergic neurons. It acts as a vasoconstrictor, the opposite of what is required for active sweating. * **B. Enkephalin:** These are endogenous opioids primarily involved in pain modulation and are found in the adrenal medulla and CNS, not typically associated with sweat gland innervation. * **C. Substance P:** While involved in neurogenic inflammation and pain transmission, it is not the primary co-transmitter for thermoregulatory sweat glands. **Clinical Pearls & High-Yield Facts:** * **Exception to the Rule:** Most sympathetic postganglionic neurons release Norepinephrine; sweat glands (and some blood vessels in skeletal muscle) are the major exceptions, releasing ACh. * **Cotransmission:** This concept describes a single nerve terminal releasing multiple transmitters. In the ANS, the ACh-VIP pair is the classic example for parasympathetic-like sympathetic fibers. * **Clinical Correlation:** In **Lambert-Eaton Syndrome** or **Botulism**, ACh release is impaired, leading to anhidrosis (lack of sweating) alongside muscle weakness.

Question 109: Synaptic conduction is mostly orthodromic because?

A. Dendrites cannot be depolarized.
B. Once repolarized, an area cannot be depolarized.
C. The strength of the antidromic impulse is less.
D. Chemical mediators are located only in the presynaptic terminal. (Correct Answer)

Explanation: **Explanation:** The directionality of synaptic transmission is governed by the **Bell-Magendie Law**, which states that impulses in a reflex arc pass in only one direction. **Why Option D is Correct:** Synaptic conduction is **orthodromic** (one-way, from presynaptic to postsynaptic neuron) because of the **asymmetrical distribution of neurotransmitters**. Chemical mediators (neurotransmitters) are stored in synaptic vesicles located exclusively within the **presynaptic terminal**. When an action potential reaches the terminal, these mediators are released into the synaptic cleft to bind with receptors on the postsynaptic membrane. Since the postsynaptic membrane lacks these neurotransmitter vesicles, it cannot initiate an impulse back toward the presynaptic neuron, ensuring unidirectional flow. **Analysis of Incorrect Options:** * **Option A:** Dendrites *can* be depolarized; they contain ligand-gated channels that generate excitatory postsynaptic potentials (EPSPs). * **Option B:** This describes the absolute refractory period. While it prevents immediate re-excitation, it does not dictate the overall anatomical direction of synaptic flow. * **Option C:** Antidromic impulses (moving toward the cell body) can occur experimentally in an axon and are not necessarily "weaker"; however, they fail to cross the synapse because there is no chemical mediator at the postsynaptic site to bridge the gap. **High-Yield Facts for NEET-PG:** * **Synaptic Delay:** The time required for transmitter release and binding (usually **0.5 ms**). It is the reason why polysynaptic reflexes are slower than monosynaptic ones. * **One-way Valve:** The synapse acts as a "biological valve." * **Electrical Synapses:** Unlike chemical synapses, these occur via **gap junctions** and can be bidirectional (lacks synaptic delay).

Question 110: Most peptide and protein hormones are synthesized as what?

A. A secretagogue
B. A pleiotropic hormone
C. Proopiomelanocortin (POMC)
D. A preprohormone (Correct Answer)

Explanation: **Explanation:** The synthesis of peptide and protein hormones follows a specific hierarchical sequence within the cell’s protein-making machinery. **Why the correct answer is right:** Most peptide hormones are initially synthesized on the ribosomes as large, inactive proteins called **preprohormones**. 1. **Preprohormone:** Contains a signal peptide (the "pre" sequence) that directs the protein to the endoplasmic reticulum (ER). 2. **Prohormone:** Once inside the ER, the signal peptide is cleaved, leaving the prohormone. 3. **Active Hormone:** The prohormone is then packaged into secretory vesicles in the Golgi apparatus, where proteolytic enzymes cleave it into the final active hormone and inactive fragments (e.g., Proinsulin is cleaved into Insulin and C-peptide). **Why the incorrect options are wrong:** * **A. Secretagogue:** This is a substance that stimulates the secretion of another substance (e.g., Gastrin is a secretagogue for gastric acid). It is a functional classification, not a structural precursor. * **B. Pleiotropic hormone:** This refers to a single hormone having multiple distinct physiological effects on different target tissues (e.g., Insulin affects liver, muscle, and adipose tissue). * **C. Proopiomelanocortin (POMC):** While POMC is a classic example of a prohormone (precursor to ACTH, MSH, and Endorphins), it is a *specific* molecule, not the general term for the synthesis stage of all peptide hormones. **High-Yield Clinical Pearls for NEET-PG:** * **C-Peptide:** Clinical marker for endogenous insulin production. Since it is cleaved from proinsulin in a 1:1 ratio with insulin, it helps distinguish Type 1 from Type 2 Diabetes. * **Storage:** Unlike steroid hormones (which are synthesized on demand), peptide hormones are stored in **secretory vesicles** and released via exocytosis in response to a stimulus (usually involving increased intracellular Ca²⁺). * **Solubility:** Peptide hormones are water-soluble and circulate freely in the plasma (except for IGF-1 and Growth Hormone, which have carrier proteins).

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