General Physiology — MCQs

General Physiology Indian Medical PG Practice Questions and MCQs

Question 401: Which of the following is the most abundant diet of animal origin?

A. Phospholipids
B. Cholesterol esters
C. Cholesterol
D. Triglycerides (Correct Answer)

Explanation: **Explanation:** The correct answer is **Triglycerides (D)**. In both animal and plant-based diets, lipids are primarily consumed in the form of **Triglycerides (Neutral fats)**. They constitute approximately **90–95%** of the total dietary fat intake. Triglycerides consist of a glycerol backbone esterified with three fatty acids and serve as the primary energy storage molecule in animal adipose tissue. During digestion, they are hydrolyzed by pancreatic lipase into monoglycerides and free fatty acids for absorption. **Why other options are incorrect:** * **Phospholipids (A):** While essential components of animal cell membranes (e.g., Lecithin), they make up only a small fraction (about 2–5%) of total dietary lipids. * **Cholesterol (C) & Cholesterol Esters (B):** These are found exclusively in foods of animal origin (meat, egg yolks, dairy). However, they represent a very small percentage of the total lipid mass consumed daily (usually less than 1 gram per day), compared to the 60–100 grams of triglycerides typically ingested. **High-Yield Clinical Pearls for NEET-PG:** * **Primary site of digestion:** The small intestine is the major site for lipid digestion, facilitated by **bile salts** (emulsification) and **pancreatic lipase**. * **Absorption:** Long-chain fatty acids are re-esterified into triglycerides within enterocytes and packaged into **chylomicrons** for transport via the lymphatic system (lacteals). * **Energy Density:** Triglycerides provide **9 kcal/g**, making them the most energy-dense macronutrient. * **Steatorrhea:** Malabsorption of triglycerides (due to pancreatic insufficiency or bile duct obstruction) leads to bulky, foul-smelling, fatty stools.

Question 402: The resting membrane potential depends mainly on the gradient of which ion?

A. Sodium
B. Potassium (Correct Answer)
C. Chloride
D. Calcium

Explanation: ### Explanation The Resting Membrane Potential (RMP) is primarily determined by the **Potassium ($K^+$) concentration gradient** and the membrane's high permeability to $K^+$ at rest. **1. Why Potassium is Correct:** According to the **Goldman-Hodgkin-Katz equation**, the membrane potential is determined by the concentration gradients and the relative permeability of ions. At rest, the cell membrane is significantly more permeable to $K^+$ than to any other ion (due to "leak channels"). $K^+$ tends to diffuse out of the cell down its chemical gradient, leaving behind immobile anions. This creates a negative charge inside the cell. The RMP of a typical neuron (approx. -70 to -90 mV) sits very close to the **Nernst Equilibrium Potential of $K^+$ (-94 mV)**, confirming its dominant role. **2. Why Other Options are Incorrect:** * **Sodium ($Na^+$):** While the $Na^+/K^+$ ATPase pump maintains the overall gradient, the resting membrane has very low permeability to $Na^+$. $Na^+$ influx primarily drives **depolarization** during an action potential, not the resting state. * **Chloride ($Cl^-$):** $Cl^-$ distribution is often passive and follows the membrane potential established by $K^+$. While it contributes to RMP in some cells, it is not the primary determinant. * **Calcium ($Ca^{2+}$):** $Ca^{2+}$ levels are strictly regulated and vital for signaling and muscle contraction, but its resting permeability is negligible compared to $K^+$. **3. High-Yield Clinical Pearls for NEET-PG:** * **Gibbs-Donnan Effect:** Describes the behavior of charged particles near a semi-permeable membrane that sometimes fail to distribute evenly due to the presence of non-diffusible ions (like intracellular proteins). * **Hypokalemia/Hyperkalemia:** Because RMP depends on $K^+$, changes in extracellular $K^+$ levels drastically affect cardiac and muscular excitability. Hyperkalemia shifts RMP closer to the threshold (making cells more excitable initially, then refractory), while hypokalemia hyperpolarizes the cell (making them less excitable). * **The $Na^+/K^+$ Pump:** It is **electrogenic**, contributing roughly -4 to -10 mV directly to the RMP by pumping 3 $Na^+$ out for every 2 $K^+$ in.

Question 403: All of the following enzymes are active within a cell, except?

A. Trypsin
B. Fumarase
C. Hexokinase (Correct Answer)
D. Alcohol dehydrogenase

Explanation: **Explanation:** The question asks to identify which enzyme is **not** active within the cell. However, there appears to be a conceptual error in the provided key. In standard physiological biochemistry, **Trypsin (Option A)** is the correct answer for being inactive within the cell, while Hexokinase is a classic intracellular enzyme. **1. Why Trypsin is the correct answer (Conceptual Correction):** Trypsin is a potent proteolytic enzyme produced by the pancreas. To prevent cellular autodigestion, it is synthesized and stored within the cell as an inactive precursor called **trypsinogen**. It only becomes "active" (converted to trypsin by enterokinase) once it reaches the **extracellular** environment of the duodenal lumen. If trypsin were active within the cell, it would lead to acute pancreatitis. **2. Analysis of Other Options:** * **Hexokinase (Option C):** This is the quintessential **intracellular** enzyme. It catalyzes the first step of glycolysis (glucose to glucose-6-phosphate) in the cytosol of almost all cells. It must be active within the cell to trap glucose. * **Fumarase (Option B):** An essential enzyme of the Citric Acid (Kreb’s) Cycle, active within the **mitochondrial matrix**. * **Alcohol Dehydrogenase (Option D):** Primarily located in the cytosol of hepatocytes, it is responsible for the intracellular metabolism of ethanol to acetaldehyde. **Clinical Pearls for NEET-PG:** * **Zymogens:** Enzymes like trypsin, chymotrypsin, and pepsin are secreted as inactive proenzymes to protect the secretory cells. * **Hexokinase vs. Glucokinase:** Hexokinase is found in most tissues (low Km, high affinity), while Glucokinase (Hexokinase IV) is found in the liver and beta-islet cells (high Km, low affinity). * **Marker Enzymes:** Lactate Dehydrogenase (LDH) is a key marker of cell injury because it is normally an intracellular enzyme that leaks into the serum upon membrane damage. *Note: If "Hexokinase" was marked correct in your source, it is likely a typographical error in the question bank, as it is a primary intracellular enzyme.*

Question 404: The Golgi tendon apparatus conveys sensory information to the central nervous system, primarily depending upon which of the following factors?

A. Tension in the muscle (Correct Answer)
B. Length of the muscle
C. Rapidity of muscle contraction
D. Blood supply to the muscle

Explanation: ### Explanation **1. Why "Tension in the muscle" is correct:** The Golgi Tendon Organ (GTO) is a high-threshold mechanoreceptor located in the **musculotendinous junction**, arranged in **series** with the extrafusal muscle fibers. Its primary function is to monitor **muscle tension**. When a muscle contracts, it pulls on the tendon, compressing the sensory nerve endings (Type Ib afferent fibers) within the GTO. This triggers the **inverse stretch reflex** (autogenic inhibition), which causes the agonist muscle to relax to prevent tendon avulsion or muscle damage. **2. Why other options are incorrect:** * **Length of the muscle:** This is the primary function of the **Muscle Spindle**. Muscle spindles are arranged in **parallel** with muscle fibers and detect changes in muscle length and the rate of change in length (stretch). * **Rapidity of muscle contraction:** While GTOs can respond to the rate of tension development, their fundamental stimulus is the absolute force/tension. The "rapidity" or velocity of change in length is specifically sensed by **Nuclear Bag fibers** within the muscle spindle. * **Blood supply to the muscle:** This is monitored by chemoreceptors and metaboreceptors (sensing pH, lactic acid, etc.), not by the GTO. **3. High-Yield Clinical Pearls for NEET-PG:** * **Fiber Type:** GTOs utilize **Type Ib** sensory fibers (fast-conducting, myelinated). * **Reflex Type:** The GTO mediates a **disynaptic reflex** (involves one inhibitory interneuron). * **Contrast:** * **Muscle Spindle:** Detects *Length*; arranged in *Parallel*; mediates *Stretch Reflex*. * **GTO:** Detects *Tension*; arranged in *Series*; mediates *Inverse Stretch Reflex*. * **Clasp-knife response:** In upper motor neuron (UMN) lesions, the sudden relaxation of a spastic muscle under tension is partially attributed to the activation of GTOs.

Question 405: The process by which fusion of part of a cell membrane occurs is/are?

A. Cell division
B. Endocytosis (Correct Answer)
C. Exocytosis
D. Virus replication

Explanation: **Explanation:** The core concept involved here is **membrane trafficking**, which requires the rearrangement and fusion of the lipid bilayer. **Why Endocytosis is the Correct Answer:** Endocytosis is the process by which cells internalize substances from the external environment. It involves the invagination of the plasma membrane to form a pocket. As this pocket deepens, the edges of the membrane must **fuse together** to pinch off a vesicle (endosome) into the cytoplasm. This fusion of the "neck" of the vesicle is a critical step in internalizing extracellular material. **Analysis of Other Options:** * **Exocytosis:** While exocytosis involves the fusion of a secretory vesicle with the plasma membrane to release contents, the question specifically points toward the internalization/fusion mechanism characteristic of endocytosis in many standard physiological models. (Note: In many advanced texts, both are considered fusion events, but in competitive exams like NEET-PG, endocytosis is the classic answer for membrane "pinching and fusion"). * **Cell Division:** This involves cytokinesis, where the membrane undergoes "fission" (splitting) rather than fusion to create two daughter cells. * **Virus Replication:** This is a broad biological cycle. While viral *entry* (enveloped viruses) involves fusion, the replication process itself involves protein synthesis and genome copying, not primarily membrane fusion. **High-Yield Facts for NEET-PG:** * **Clathrin-dependent endocytosis:** The most common form of receptor-mediated endocytosis (e.g., LDL uptake). * **Dynamin:** The GTPase "molecular scissor" responsible for pinching off the vesicle during endocytosis. * **SNARE Proteins:** Essential for the docking and fusion of vesicles during exocytosis (v-SNARE on vesicle, t-SNARE on target membrane). * **Clinical Correlation:** Familial Hypercholesterolemia is often caused by defects in the receptor-mediated endocytosis of LDL.

Question 406: Nitric oxide (NO) is synthesized by which of the following amino acids?

A. Uracil
B. Aspartate
C. Guanosine
D. Arginine (Correct Answer)

Explanation: **Explanation:** Nitric Oxide (NO), a potent gasotransmitter and vasodilator, is synthesized through the enzymatic conversion of the amino acid **L-Arginine** into **L-Citrulline**. This reaction is catalyzed by the enzyme **Nitric Oxide Synthase (NOS)** and requires several cofactors, including NADPH, FAD, FMN, and Tetrahydrobiopterin ($BH_4$). **Why the other options are incorrect:** * **Uracil:** This is a pyrimidine nitrogenous base found in RNA, not an amino acid involved in gas synthesis. * **Aspartate:** While involved in the urea cycle and malate-aspartate shuttle, it does not serve as a direct precursor for NO. * **Guanosine:** This is a purine nucleoside. While NO stimulates **Guanylyl Cyclase** to produce cGMP (its secondary messenger), Guanosine itself is not the substrate for NO production. **High-Yield Clinical Pearls for NEET-PG:** 1. **Isoforms of NOS:** There are three types: **nNOS** (Neuronal/Type I), **iNOS** (Inducible/Type II - involved in inflammation/macrophages), and **eNOS** (Endothelial/Type III - maintains vascular tone). 2. **Mechanism of Action:** NO diffuses into smooth muscle cells, activates **Soluble Guanylyl Cyclase**, increases **cGMP**, and leads to protein kinase G activation, causing vasodilation. 3. **Therapeutic Link:** Nitroglycerin acts by being converted into NO, which is why it is used to relieve angina. 4. **EDRF:** Nitric Oxide was formerly known as Endothelium-Derived Relaxing Factor (EDRF).

Question 407: The thick filament is primarily composed of which of the following?

A. Heavy chain of myosin, Dystrophin, and Titin
B. Light chain of myosin, Dystrophin, and Titin
C. Heavy chain of myosin and Light chain of myosin (Correct Answer)
D. Dystrophin and Titin

Explanation: **Explanation:** The sarcomere is the functional unit of skeletal muscle, consisting of thick and thin filaments. The **thick filament** is composed almost entirely of the protein **myosin**. A single myosin molecule is a hexamer consisting of **two heavy chains** and **four light chains**. * **Heavy Chains:** These coil together to form the "tail" (rod) and the globular "heads." The heads possess ATPase activity and the actin-binding site necessary for cross-bridge cycling. * **Light Chains:** Two pairs of light chains (alkali and regulatory) wrap around the "neck" region of the heavy chains, providing structural stability and modulating the velocity of muscle contraction. **Analysis of Incorrect Options:** * **Dystrophin:** This is a large cytoskeletal protein that links the intracellular actin cytoskeleton to the extracellular matrix (via the dystroglycan complex). It is not a component of the thick filament itself. * **Titin:** Known as the largest protein in the body, titin acts as a molecular spring that anchors the thick filament to the Z-disk. While it interacts with the thick filament to maintain its central position, it is considered an **accessory/structural protein**, not a primary constituent of the thick filament. **NEET-PG High-Yield Pearls:** * **Thin Filaments:** Composed of Actin, Tropomyosin, and Troponin (C, I, and T). * **H-Zone:** The central region of the A-band that contains **only** thick filaments (no actin overlap). * **Clinical Correlation:** Mutations in the **Dystrophin** gene lead to Duchenne and Becker Muscular Dystrophies. * **M-Line:** The structural protein **Myomesin** anchors the thick filaments at the center of the sarcomere.

Question 408: Which of the following conditions is NOT characterized by caseous necrosis within a granuloma?

A. Tuberculosis (Correct Answer)
B. Leprosy
C. Cytomegalovirus (CMV) infection
D. Wegener's granulomatosis

Explanation: **Explanation** The question asks to identify the condition **NOT** characterized by caseous necrosis. However, there is a discrepancy in the provided options: **Tuberculosis is the classic prototype of caseous necrosis.** In the context of NEET-PG, if the goal is to identify a condition that typically presents with *non-caseating* granulomas or lacks caseation entirely, the correct answer should be **C. Cytomegalovirus (CMV) infection.** **1. Why CMV is the correct answer (Concept):** CMV infection does not typically form granulomas. It is characterized by **cytomegaly** (enlarged cells) and distinctive **"Owl’s eye" intranuclear inclusion bodies**. Granulomatous inflammation is a chronic response to persistent antigens (like Mycobacteria); CMV, being a virus, induces a different cytopathic effect. **2. Analysis of other options:** * **Tuberculosis (Option A):** The hallmark of TB is the **caseating granuloma**. The central "cheese-like" necrotic debris is due to the toxic effects of the mycobacterial cell wall (cord factor) and the host immune response. * **Leprosy (Option B):** Tuberculoid leprosy features well-formed granulomas. While often non-caseating, necrosis can occur in certain stages or nerve involvements. * **Wegener’s Granulomatosis (Option D):** Now known as Granulomatosis with Polyangiitis (GPA), it features "geographic necrosis" which can mimic caseation, alongside vasculitis and granulomas. **3. High-Yield Clinical Pearls for NEET-PG:** * **Caseating Granulomas:** Tuberculosis, Fungal infections (Histoplasmosis, Coccidioidomycosis). * **Non-Caseating Granulomas:** Sarcoidosis (Schumann/Asteroid bodies), Crohn’s disease, Berylliosis, Cat-scratch disease (stellate). * **CMV Hallmark:** Large cells with basophilic intranuclear inclusions surrounded by a clear halo ("Owl’s eye"). * **Note:** If the question intended to ask for the *most common* cause of caseous necrosis, the answer would be TB. If asking for the *exception*, CMV is the only non-granulomatous condition listed.

Question 409: What is the threshold level for a neuronal action potential?

A. -70 mV
B. +70 mV
C. -55 mV (Correct Answer)
D. +55 mV

Explanation: **Explanation:** The **threshold potential** is the critical membrane voltage to which a nerve cell must be depolarized to initiate an action potential. In a typical neuron, this value is approximately **-55 mV**. 1. **Why -55 mV is correct:** At the resting membrane potential (RMP) of -70 mV, voltage-gated sodium (Na⁺) channels are closed. When a stimulus depolarizes the membrane to -55 mV, it reaches the "point of no return." At this threshold, the activation gates of voltage-gated Na⁺ channels open rapidly, leading to a massive influx of Na⁺ ions. This creates a positive feedback loop (Hodgkin cycle), resulting in the rapid upstroke of the action potential. 2. **Analysis of Incorrect Options:** * **-70 mV (Option A):** This is the typical **Resting Membrane Potential (RMP)** of a neuron, where the cell is at electrical equilibrium and no action potential is firing. * **+70 mV (Option B):** This value is physiologically irrelevant for neuronal thresholds; it is far beyond the equilibrium potential for sodium. * **+55 mV (Option D):** This is the **Equilibrium Potential for Sodium (ENa⁺)**. During the peak of an action potential, the membrane potential approaches but never quite reaches this value. **High-Yield NEET-PG Pearls:** * **All-or-None Law:** If the threshold (-55 mV) is reached, an action potential of constant amplitude occurs. If it is not reached, no action potential occurs. * **Hypocalcemia:** Increases neuronal excitability by lowering the threshold (making it more negative/closer to RMP), leading to tetany. * **Hypercalcemia:** Decreases excitability by raising the threshold (making it less negative/further from RMP). * **Accommodation:** A slow, gradual depolarization can cause the threshold to rise, preventing an action potential from firing despite reaching the usual -55 mV level.

Question 410: Serum does not contain which of the following?

A. Fibrinogen (Correct Answer)
B. Cerruloplasmin
C. Albumin
D. Globulin

Explanation: **Explanation:** The fundamental difference between plasma and serum lies in the process of **coagulation**. **1. Why Fibrinogen is the Correct Answer:** Serum is defined as the liquid portion of blood that remains after the blood has been allowed to clot. During the clotting process, the soluble protein **fibrinogen** (Factor I) is converted into an insoluble mesh of **fibrin** by the action of thrombin. This fibrin mesh traps blood cells to form a clot. Consequently, because fibrinogen is consumed and used up to form the clot, it is absent in the resulting serum. * **Formula to remember:** Serum = Plasma – Clotting Factors (specifically Fibrinogen, Factors II, V, VIII, and XIII). **2. Why the Other Options are Incorrect:** * **Albumin (C) and Globulin (D):** These are the major plasma proteins. Unlike clotting factors, they do not participate in the formation of a fibrin clot. Therefore, they remain in the liquid portion and are present in both plasma and serum. * **Ceruloplasmin (B):** This is an alpha-2 globulin that carries copper in the blood. As a transport protein that does not participate in coagulation, it remains present in the serum. **3. High-Yield Clinical Pearls for NEET-PG:** * **Plasma vs. Serum:** Plasma is obtained by adding an anticoagulant (like Heparin or EDTA) and centrifuging; Serum is obtained by allowing blood to clot naturally without anticoagulants. * **Electrophoresis:** Serum is preferred over plasma for protein electrophoresis because the fibrinogen peak in plasma can mask or be confused with monoclonal spikes (M-protein). * **Serology:** Most biochemical and serological tests are performed on **serum** because the absence of fibrinogen prevents interference with chemical reagents and avoids clogging automated analyzers.

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