General Physiology — MCQs

General Physiology Indian Medical PG Practice Questions and MCQs

Question 11: Which of the following is NOT an example of a feedforward control system?

A. Temperature regulation
B. Blood pressure regulation (Correct Answer)
C. Cephalic phase of gastric secretion
D. Increased heart rate before the start of exercise

Explanation: ### Explanation In physiology, control systems are categorized into **Negative Feedback**, **Positive Feedback**, and **Feedforward** mechanisms. **Why Blood Pressure Regulation is the Correct Answer:** Blood pressure regulation (via the Baroreceptor reflex) is a classic example of a **Negative Feedback System**. When blood pressure rises, baroreceptors detect the stretch and trigger responses to lower it back to the set point. Feedforward control, by contrast, is "anticipatory"—it initiates a response *before* a change in the variable actually occurs to prevent a disturbance. Since blood pressure regulation reacts to an existing change rather than anticipating one, it is not a feedforward system. **Analysis of Incorrect Options (Feedforward Examples):** * **Temperature Regulation:** The body has central and peripheral thermoreceptors. When we move into a cold environment, skin receptors signal the brain to initiate shivering and vasoconstriction *before* the core body temperature actually drops. * **Cephalic Phase of Gastric Secretion:** The sight, smell, or thought of food triggers the vagus nerve to stimulate gastric acid secretion in anticipation of a meal, preparing the stomach before food arrives. * **Increased Heart Rate Before Exercise:** Known as the "Anticipatory Rise," the cerebral cortex sends signals to the medulla to increase heart rate and ventilation before physical exertion begins, ensuring oxygen delivery meets the upcoming demand. **High-Yield Clinical Pearls for NEET-PG:** * **Feedforward Control:** Also known as "adaptive control," it minimizes the delay inherent in feedback systems. * **Negative Feedback:** The most common homeostatic mechanism (e.g., hormone regulation, pH balance). * **Positive Feedback:** Rare and often leads to an "instability" or "vicious cycle" (e.g., LH surge, Oxytocin in labor, Blood clotting, Nerve action potential). * **Error Signal:** In feedforward systems, the controller uses sensory information to predict an error and correct it before it happens.

Question 12: The Monro-Kellie doctrine is related to the doctrine of non-compressible contents within which part of the body?

A. Head (Correct Answer)
B. Abdomen
C. Chest
D. Leg

Explanation: ### Explanation **The Monro-Kellie Doctrine (Hypothesis)** The correct answer is **Head (A)**. The Monro-Kellie doctrine states that the cranial vault is a rigid, non-expandable container with a fixed total volume. This volume is composed of three nearly incompressible components: 1. **Brain parenchyma** (~80%) 2. **Cerebrospinal fluid (CSF)** (~10%) 3. **Blood** (~10%) According to this principle, because the total volume is constant, an increase in any one of these components (or the addition of a pathological mass like a tumor or hematoma) must be compensated by an equal decrease in the volume of another. If compensation fails, **Intracranial Pressure (ICP)** rises sharply. **Why the other options are incorrect:** * **B. Abdomen & C. Chest:** Unlike the skull, the abdominal and thoracic cavities have flexible walls (diaphragm, intercostal muscles, and skin) that can expand to accommodate volume changes. * **D. Leg:** The limbs are not enclosed in a rigid bony vault; while they can experience compartment syndrome, they do not follow the specific volumetric constraints of the Monro-Kellie doctrine. **High-Yield Clinical Pearls for NEET-PG:** * **Primary Compensatory Mechanisms:** When ICP rises, the body first displaces **CSF** into the spinal subarachnoid space and shunts **venous blood** out of the skull. * **Cushing’s Triad:** A late sign of increased ICP resulting from the Monro-Kellie doctrine failure, characterized by **Hypertension, Bradycardia, and Irregular Respiration.** * **Clinical Application:** This doctrine explains why even small intracranial hemorrhages can lead to rapid neurological deterioration and brain herniation.

Question 13: Inward flow of Na+ in heart leads to what?

A. Plateau phase
B. Action potential (Correct Answer)
C. Repolarization
D. No change

Explanation: ### Explanation **Correct Answer: B. Action potential** In cardiac physiology, the **inward flow of Sodium (Na+)** ions through fast voltage-gated sodium channels is the primary driver for **Phase 0 (Depolarization)** of the action potential in non-pacemaker cells (atrial and ventricular myocytes). When the cell membrane reaches its threshold potential, these channels open rapidly, allowing a massive influx of Na+ down its electrochemical gradient. This sudden shift in membrane potential from negative to positive initiates the cardiac action potential. **Analysis of Incorrect Options:** * **A. Plateau phase:** This is **Phase 2** of the cardiac action potential. It is primarily maintained by the **inward flow of Calcium (Ca2+)** through L-type calcium channels, balanced by the outward flow of Potassium (K+). * **C. Repolarization:** This occurs during **Phases 1, 2, and 3**. The main driver for repolarization (returning the cell to a negative state) is the **outward flow of Potassium (K+)** ions. * **D. No change:** Inward Na+ flux causes a significant electrical change (depolarization), making this option incorrect. **High-Yield Clinical Pearls for NEET-PG:** * **Fast Response vs. Slow Response:** Fast Na+ channels are responsible for depolarization in myocytes and Purkinje fibers. In contrast, the **SA and AV nodes** (pacemaker cells) lack functional fast Na+ channels; their depolarization is driven by **inward Calcium (Ca2+)** flow. * **Class I Antiarrhythmics:** Drugs like Lidocaine or Flecainide work by blocking these fast Na+ channels, thereby slowing the rate of Phase 0 depolarization. * **Tetrodotoxin (TTX):** A potent toxin that specifically inhibits these voltage-gated Na+ channels, preventing action potential generation.

Question 14: Concurrent flexion of both wrists in response to electrical stimulation is characteristic of which area of the nervous system?

A. Postcentral gyrus
B. Vestibulospinal tract
C. Dentate nucleus
D. Supplementary motor cortex (Correct Answer)

Explanation: **Explanation:** The **Supplementary Motor Area (SMA)**, located on the medial surface of the frontal lobe (Brodmann area 6), is primarily involved in the planning and coordination of complex, bilateral movements. Unlike the Primary Motor Cortex (M1), which controls discrete muscles on the contralateral side, electrical stimulation of the SMA typically results in **bilateral, synergistic movements** or postural adjustments, such as the concurrent flexion of both wrists or the raising of both arms. **Analysis of Options:** * **A. Postcentral Gyrus:** This is the Primary Somatosensory Cortex (S1). Stimulation here results in sensory perceptions (tingling, numbness) on the contralateral side of the body, not motor movements. * **B. Vestibulospinal Tract:** This is an extrapyramidal pathway responsible for maintaining equilibrium and muscle tone (primarily extensors) in response to head movements. It does not coordinate complex bilateral wrist flexion. * **C. Dentate Nucleus:** The largest of the deep cerebellar nuclei, it is involved in the planning and timing of movements. While it influences motor output, direct electrical stimulation does not produce the specific bilateral motor patterns characteristic of the SMA. **High-Yield Facts for NEET-PG:** * **SMA vs. Premotor Cortex:** SMA is for **internal cues** (complex sequences from memory), while the Premotor cortex is for **external cues** (sensory-guided movements). * **Jacksonian March:** Associated with the Primary Motor Cortex (M1), not the SMA. * **Alien Hand Syndrome:** Can occur with lesions involving the SMA and corpus callosum. * **Stimulation Threshold:** The SMA has a higher threshold for electrical stimulation compared to the Primary Motor Cortex.

Question 15: The repolarization phase of an action potential is due to which of the following ionic changes?

A. Increasing sodium permeability and increasing potassium permeability
B. Increasing sodium permeability and decreasing potassium permeability
C. Decreasing sodium permeability and an immediate increasing potassium permeability
D. Decreasing sodium permeability and a delayed increase in potassium permeability (Correct Answer)

Explanation: ### Explanation The action potential is a rapid change in membrane potential characterized by sequential changes in ion conductance. **Why Option D is Correct:** The repolarization phase occurs due to two simultaneous events triggered at the peak of the action potential: 1. **Inactivation of Voltage-Gated Na⁺ Channels:** The "h-gates" (inactivation gates) of sodium channels close, leading to a **decrease in sodium permeability**. This stops the inward flow of positive charge. 2. **Activation of Voltage-Gated K⁺ Channels:** Unlike Na⁺ channels which open rapidly, K⁺ channels are "slow" to open. This **delayed increase in potassium permeability** allows K⁺ to exit the cell down its electrochemical gradient, restoring the negative resting membrane potential. **Analysis of Incorrect Options:** * **Option A & B:** These are incorrect because sodium permeability must **decrease** (inactivate) for repolarization to occur. Continued sodium influx would maintain depolarization. * **Option C:** While sodium permeability does decrease, the increase in potassium permeability is not "immediate." The delay in K⁺ channel opening is a fundamental physiological property that ensures the action potential reaches its peak before repolarization begins. **High-Yield NEET-PG Pearls:** * **Refractory Period:** The absolute refractory period is primarily due to the **inactivation of Na⁺ channels**. * **Hyperpolarization:** The "delayed" nature of K⁺ channels also causes them to be slow to close, leading to the **after-hyperpolarization** phase. * **Tetrodotoxin (TTX):** A potent toxin that blocks voltage-gated Na⁺ channels, preventing depolarization. * **TEA (Tetraethylammonium):** Blocks voltage-gated K⁺ channels, specifically inhibiting the repolarization phase.

Question 16: The electrical potential difference necessary for a single ion to be at equilibrium across a membrane is best described by which equation?

A. Goldman equation
B. van't Hoff equation
C. Fick's law
D. Nernst equation (Correct Answer)

Explanation: **Explanation:** The correct answer is the **Nernst equation**. **1. Why the Nernst Equation is Correct:** The Nernst equation calculates the **equilibrium potential (electrochemical equilibrium)** for a **single ion**. It determines the electrical potential difference across a cell membrane that exactly balances the concentration gradient of that specific ion, resulting in no net movement of the ion across the membrane. * **Formula:** $E = \frac{RT}{zF} \ln \frac{[Ion]_{outside}}{[Ion]_{inside}}$ (Simplified at body temperature: $E = \frac{61}{z} \log \frac{[C]_{out}}{[C]_{in}}$). **2. Why Other Options are Incorrect:** * **Goldman-Hodgkin-Katz (GHK) Equation:** Unlike the Nernst equation, this accounts for **multiple ions** (Na⁺, K⁺, Cl⁻) simultaneously and considers their relative **membrane permeability**. It is used to calculate the actual Resting Membrane Potential (RMP). * **van't Hoff Equation:** This is used to calculate **osmotic pressure** ($\pi = MRT$). It relates the concentration of solutes to the pressure required to stop osmosis. * **Fick’s Law:** This describes the **rate of diffusion** of a gas or solute across a membrane. It states that the flux is proportional to the concentration gradient and surface area, but inversely proportional to membrane thickness. **3. High-Yield Clinical Pearls for NEET-PG:** * **Standard Equilibrium Potentials:** $E_{K^+}$ is approx. **-94 mV**; $E_{Na^+}$ is approx. **+61 mV**. * The Resting Membrane Potential (-70 to -90 mV) is closest to $E_{K^+}$ because the membrane is most permeable to Potassium at rest (via leak channels). * If permeability to an ion increases (e.g., Na⁺ during depolarization), the membrane potential shifts toward that ion's Nernst potential.

Question 17: Total lung capacity depends on which of the following?

A. Size of airway
B. Closing tidal volume
C. Lung compliance (Correct Answer)
D. Residual volume

Explanation: **Explanation:** **Total Lung Capacity (TLC)** is the maximum volume of air the lungs can hold after a maximal inspiratory effort. It is determined by the balance between the outward pull of the inspiratory muscles and the inward elastic recoil of the lungs and chest wall. **Why Lung Compliance is Correct:** Compliance refers to the "distensibility" or the ease with which the lungs expand under pressure. * **High Compliance:** In conditions like **Emphysema**, the loss of elastic tissue makes the lungs overly distensible, leading to an **increased TLC**. * **Low Compliance:** In **Restrictive Lung Diseases** (e.g., Pulmonary Fibrosis), the lungs become stiff and resist expansion, significantly **decreasing TLC**. Therefore, the elastic properties (compliance) are a primary determinant of the lung's maximum volume. **Analysis of Incorrect Options:** * **Size of Airway:** This primarily affects **airway resistance** and flow rates (like FEV1), not the total volume capacity of the lung parenchyma. * **Closing Tidal Volume:** This relates to the point during expiration when small airways in the dependent parts of the lung begin to close. It is a measure of airway stability, not total capacity. * **Residual Volume (RV):** While RV is a *component* of TLC (TLC = VC + RV), it is a sub-volume rather than a physiological determinant. TLC is the independent variable defined by the limits of expansion. **High-Yield Clinical Pearls for NEET-PG:** * **TLC Formula:** TLC = Vital Capacity (VC) + Residual Volume (RV). * **Helium Dilution/Body Plethysmography:** These are required to measure TLC, as spirometry cannot measure RV. * **Emphysema Paradox:** TLC increases due to high compliance, but effective gas exchange decreases due to alveolar destruction. * **Scoliosis/Kyphosis:** These decrease TLC by reducing **chest wall compliance**.

Question 18: All of the following statements about Hepcidin are true EXCEPT:

A. It is a 25-amino acid peptide secreted by Kupffer cells.
B. It decreases duodenal absorption of iron.
C. It regulates the activity of transporters DMT-1 and ferroportin-1.
D. Mice with enhanced hepcidin expression have elevated body iron stores. (Correct Answer)

Explanation: **Explanation:** Hepcidin is the master regulator of iron homeostasis in the human body. Understanding its mechanism is crucial for NEET-PG. **1. Why Option D is the Correct Answer (The False Statement):** Hepcidin **inhibits** iron entry into the plasma. It binds to **Ferroportin**, causing its internalization and degradation. Since Ferroportin is the only known iron exporter, high levels of Hepcidin prevent iron release from macrophages and enterocytes. Therefore, mice with enhanced Hepcidin expression would suffer from **iron-deficiency anemia** and have **low** body iron stores, not elevated ones. **2. Analysis of Other Options:** * **Option A:** Hepcidin is indeed a 25-amino acid peptide. While primarily produced by **hepatocytes**, it is also associated with the liver's reticuloendothelial system (Kupffer cells) in response to inflammation. * **Option B:** By degrading Ferroportin on the basolateral membrane of enterocytes, Hepcidin effectively blocks the transfer of dietary iron into the blood, thus decreasing duodenal absorption. * **Option C:** Hepcidin primarily targets Ferroportin-1. However, it also indirectly regulates **DMT-1** (Divalent Metal Transporter 1) by reducing its expression on the apical membrane, further limiting iron uptake. **3. High-Yield Clinical Pearls for NEET-PG:** * **Stimulus:** Hepcidin synthesis is increased by **Iron overload** and **Inflammation (IL-6)**. It is decreased by hypoxia and increased erythropoietic activity. * **Anemia of Chronic Disease:** Driven by high Hepcidin levels (due to IL-6), leading to iron sequestration in macrophages. * **Hemochromatosis:** Often caused by Hepcidin deficiency or resistance, leading to uncontrolled iron absorption. * **Mnemonic:** Hepcidin **"Hides"** iron (keeps it inside cells and out of the blood).

Question 19: Which muscle fiber type has a high glycogen content?

A. Red fibers
B. Type 1 fibers
C. White fibers (Correct Answer)
D. Tonic fibers

Explanation: **Explanation:** The classification of muscle fibers is based on their metabolic profile and contraction speed. **White fibers (Type IIb/IIx)** are designed for short bursts of high-intensity activity (anaerobic power). Because they have fewer mitochondria and less myoglobin, they rely primarily on **anaerobic glycolysis** for ATP production. To support this rapid energy demand, they store significantly **high amounts of glycogen** and have high glycolytic enzyme activity. **Analysis of Options:** * **White fibers (Correct):** These are "Fast-twitch" fibers. They have high glycogen content, high myosin ATPase activity, and fatigue rapidly. * **Red fibers / Type 1 fibers (Incorrect):** These are "Slow-twitch" fibers. They are rich in myoglobin and mitochondria, relying on aerobic oxidative phosphorylation. They have high triglyceride (fat) stores rather than high glycogen stores and are resistant to fatigue. * **Tonic fibers (Incorrect):** These are specialized fibers (like those in extraocular muscles) that do not follow the "all-or-none" law and have multiple nerve endings. They are not characterized by high glycogen content. **High-Yield NEET-PG Pearls:** 1. **Myoglobin Content:** Type 1 (Red) = High; Type 2 (White) = Low. 2. **Mitochondria/Capillaries:** Type 1 = High (for oxygen utilization); Type 2 = Low. 3. **ATPase Activity:** Type 2 fibers have high myosin ATPase activity, leading to faster contraction. 4. **Fatigability:** Type 1 is fatigue-resistant (postural muscles); Type 2 is easily fatigued (sprinter's muscles). 5. **Intermediate Fibers:** Type IIa fibers are "Fast-Oxidative Glycolytic," sharing characteristics of both types.

Question 20: Pantothenic acid is associated with which of the following moieties?

A. coenzyme A (Correct Answer)
B. Carboxyl
C. Hydroxyl
D. H+

Explanation: **Explanation:** **Pantothenic acid (Vitamin B5)** is a water-soluble vitamin that serves as an essential precursor for the synthesis of **Coenzyme A (CoA)**. 1. **Why Coenzyme A is correct:** Pantothenic acid consists of pantoic acid and $\beta$-alanine. In the body, it is phosphorylated and combined with ATP and cysteine to form Coenzyme A. CoA is a vital cofactor for the metabolism of carbohydrates, lipids, and proteins. It acts as an acyl group carrier (forming Acetyl-CoA), which is the entry point for the TCA cycle and is crucial for fatty acid synthesis and oxidation. 2. **Why the other options are incorrect:** * **Carboxyl (B):** Biotin (Vitamin B7) is the cofactor associated with carboxylation reactions (e.g., Pyruvate carboxylase). * **Hydroxyl (C):** Vitamin C (Ascorbic acid) is the primary vitamin associated with hydroxylation, specifically of proline and lysine residues in collagen synthesis. * **H+ (D):** Niacin (B3) and Riboflavin (B2) act as hydrogen/electron carriers in the form of NAD+/NADH and FAD/FADH2. **High-Yield Clinical Pearls for NEET-PG:** * **Active Form:** The active form of Vitamin B5 is Coenzyme A and **Acyl Carrier Protein (ACP)**. * **Deficiency:** Extremely rare but manifests as **"Burning Feet Syndrome"** (Gopalan’s syndrome), characterized by paresthesia and burning sensations in the lower extremities. * **Key Reaction:** It is a component of the **Pyruvate Dehydrogenase Complex**, which converts Pyruvate to Acetyl-CoA. * **Mnemonic:** B5 makes the "A" (Coenzyme A).

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