General Physiology — MCQs

General Physiology Indian Medical PG Practice Questions and MCQs

Question 121: Fever increases the basal metabolic rate by approximately what percentage for each 1-degree Celsius rise in body temperature?

A. 3%
B. 7% (Correct Answer)
C. 10%
D. 20%

Explanation: **Explanation:** The correct answer is **7% (Option B)**. This relationship is based on the **Van’t Hoff’s Law**, which states that the velocity of chemical reactions increases as temperature rises. In human physiology, an increase in body temperature accelerates enzymatic activity and metabolic processes. For every **1°C rise** in body temperature, the Basal Metabolic Rate (BMR) increases by approximately **7%** (or roughly 13% for every 1°F rise). **Analysis of Options:** * **Option A (3%):** This value is too low. While some minor physiological fluctuations occur, a 1°C rise triggers a more significant metabolic demand than 3%. * **Option B (7%):** **Correct.** This is the standard physiological constant cited in major textbooks (like Guyton and Hall) regarding the thermogenic effect of fever on metabolism. * **Option C (10%):** While some older texts or specific clinical scenarios might approximate 10%, 7% is the precise "high-yield" figure required for competitive exams like NEET-PG. * **Option D (20%):** This is an overestimation. A 20% increase per degree would lead to rapid exhaustion of energy stores and severe metabolic acidosis during common febrile illnesses. **Clinical Pearls for NEET-PG:** * **Q10 Effect:** This refers to the temperature coefficient, representing the factor by which a biological process increases with a 10°C rise. * **Clinical Significance:** Fever-induced BMR increase explains why patients with infections experience rapid weight loss, increased heart rate (tachycardia), and increased respiratory rate (tachypnea). * **Thyroid Link:** BMR is primarily regulated by **Thyroxine (T4)**. While fever increases BMR acutely via temperature, T4 increases it by stimulating Na+-K+ ATPase activity across tissues.

Question 122: Which of the following is NOT a second messenger?

A. cAMP
B. NO
C. CO
D. Protein kinase (Correct Answer)

Explanation: **Explanation:** In cellular signaling, **second messengers** are small intracellular molecules or ions that relay signals received by cell-surface receptors (the first messenger) to target effector proteins. **Why Protein Kinase is the correct answer:** Protein kinases (e.g., PKA, PKC) are **enzymes**, not second messengers. They are typically the **effectors** activated by second messengers. For example, cAMP (second messenger) binds to and activates Protein Kinase A. The kinase then phosphorylates specific proteins to elicit a cellular response. Therefore, it is a downstream mediator in the signaling cascade rather than a messenger molecule itself. **Analysis of Incorrect Options:** * **cAMP (Cyclic Adenosine Monophosphate):** The most classic second messenger, produced by Adenylyl Cyclase. It mediates responses for hormones like Glucagon and PTH. * **NO (Nitric Oxide):** A unique gaseous second messenger that diffuses across membranes to activate soluble guanylyl cyclase, increasing cGMP levels. * **CO (Carbon Monoxide):** Similar to NO, CO acts as a gaseous signaling molecule (gasotransmitter) that can activate guanylyl cyclase and modulate ion channels. **High-Yield NEET-PG Pearls:** * **Common Second Messengers:** cAMP, cGMP, $IP_3$, DAG, $Ca^{2+}$, NO, and CO. * **Gaseous Messengers:** NO, CO, and $H_2S$ (Hydrogen Sulfide). * **IP3/DAG Pathway:** Phospholipase C cleaves $PIP_2$ into $IP_3$ (releases $Ca^{2+}$ from ER) and DAG (activates Protein Kinase C). * **Receptor Tyrosine Kinases (RTK):** Insulin and Growth Factors use this pathway, which often bypasses traditional second messengers by using direct protein-protein phosphorylation (e.g., JAK-STAT).

Question 123: Which one of the following molecules is used for cell signaling?

A. CO2
B. O2
C. NO (Correct Answer)
D. N2

Explanation: **Explanation:** The correct answer is **NO (Nitric Oxide)**. **1. Why NO is the correct answer:** Nitric Oxide (NO) is a unique **gaseous signaling molecule** (gasotransmitter). In the cardiovascular system, it is synthesized from **L-arginine** by the enzyme **Nitric Oxide Synthase (NOS)** in endothelial cells. Being lipophilic, it diffuses across cell membranes into adjacent smooth muscle cells. There, it activates **soluble Guanylyl Cyclase (sGC)**, increasing intracellular **cGMP** levels. This leads to protein kinase G activation and calcium sequestration, resulting in **vasodilation**. **2. Why the other options are incorrect:** * **CO2 (Carbon Dioxide) & O2 (Oxygen):** These are metabolic gases involved in respiration and oxidative phosphorylation. While their concentrations can influence physiological processes (e.g., chemoreceptor activation or hypoxic vasoconstriction), they do not function as primary ligands or intracellular signaling molecules in the classical sense. * **N2 (Nitrogen):** This is an inert gas in the human body. It does not participate in biochemical signaling pathways. **3. High-Yield Clinical Pearls for NEET-PG:** * **Mechanism of Nitroglycerin:** It acts as a prodrug that is converted into NO, causing rapid vasodilation in angina pectoris. * **Sildenafil (Viagra):** It inhibits **Phosphodiesterase-5 (PDE-5)**, the enzyme that breaks down cGMP. By prolonging cGMP levels initiated by NO, it maintains smooth muscle relaxation. * **NOS Isoforms:** * **eNOS** (Endothelial) and **nNOS** (Neuronal) are constitutive (calcium-dependent). * **iNOS** (Inducible) is found in macrophages and is involved in inflammation and septic shock. * **Other Gasotransmitters:** Besides NO, **Carbon Monoxide (CO)** and **Hydrogen Sulfide (H2S)** are also recognized as signaling molecules.

Question 124: Which of the following statements regarding the homeostatic mechanism of the body is NOT true?

A. Normal cell function depends on the constancy of extracellular fluid (ECF).
B. The actual environment of the cells of the body is the interstitial component of the ECF.
C. Renal and respiratory adjustments are constantly occurring to maintain homeostasis.
D. The body's homeostatic systems are primarily stabilized by positive feedback mechanisms. (Correct Answer)

Explanation: **Explanation** The concept of **Homeostasis**, first coined by Walter Cannon, refers to the maintenance of a nearly constant internal environment (*milieu intérieur*). **Why Option D is the Correct Answer (The False Statement):** The body’s homeostatic systems are primarily stabilized by **negative feedback mechanisms**, not positive feedback. Negative feedback works by initiating responses that counteract a stimulus (e.g., if blood pressure rises, the body acts to lower it). In contrast, **positive feedback** is inherently unstable because it amplifies the initial stimulus, leading to a "vicious cycle." While essential for specific physiological events (like the LH surge, blood clotting, or uterine contractions during childbirth), it is not the primary mechanism for general stability. **Analysis of Other Options:** * **Option A:** True. Cells require a stable environment (pH, temperature, osmolarity) to function. Deviations in the ECF lead to cellular dysfunction or death. * **Option B:** True. Claude Bernard’s *milieu intérieur* specifically refers to the **Extracellular Fluid (ECF)**. Since the interstitial fluid directly bathes the cells, it represents their immediate actual environment. * **Option C:** True. The kidneys (regulating electrolytes/acid-base) and lungs (regulating $CO_2$ and $O_2$) are the two most critical organs for maintaining the constancy of the ECF. **High-Yield NEET-PG Pearls:** * **Gain of a System:** The degree of effectiveness with which a control system maintains homeostasis is called "Gain." (Formula: $Gain = Correction / Error$). * **Feed-forward Control:** Occurs when the body anticipates a change before it happens (e.g., cephalic phase of gastric secretion or increased heart rate before exercise). * **Adaptive Control:** A delayed form of negative feedback used by the cerebellum to correct rapid movements.

Question 125: Energy is required for which transport process across the cell membrane?

A. Osmosis
B. Facilitated diffusion
C. Active transport (Correct Answer)
D. Simple diffusion

Explanation: **Explanation:** Transport across the cell membrane is broadly classified into **Passive** and **Active** transport based on energy requirements. **Why Active Transport is Correct:** Active transport is the movement of molecules or ions **against** a concentration or electrochemical gradient (from low to high concentration). Because this process moves "uphill," it requires the expenditure of metabolic energy, typically in the form of **ATP**. * **Primary Active Transport:** Directly uses ATP (e.g., Na⁺-K⁺ ATPase pump). * **Secondary Active Transport:** Uses the energy stored in an electrochemical gradient created by primary active transport (e.g., SGLT-1 for glucose absorption). **Why Other Options are Incorrect:** * **A. Osmosis:** This is the passive movement of water molecules through a semi-permeable membrane from a region of low solute concentration to high solute concentration. It requires no energy. * **B. Facilitated Diffusion:** This uses carrier proteins or channels to move large or polar molecules (like glucose via GLUT) down their concentration gradient. While it requires a "helper" protein, it does **not** require energy. * **D. Simple Diffusion:** This is the spontaneous movement of lipid-soluble or small molecules (like O₂ and CO₂) directly through the lipid bilayer down a concentration gradient without energy or carriers. **High-Yield Clinical Pearls for NEET-PG:** * **Na⁺-K⁺ ATPase:** The most important primary active transporter; it pumps **3 Na⁺ out** and **2 K⁺ in**, maintaining the resting membrane potential. It is inhibited by **Cardiac Glycosides (Digoxin)**. * **Saturation Kinetics:** Both Facilitated Diffusion and Active Transport exhibit "Vmax" (saturation) because they rely on carrier proteins, unlike Simple Diffusion. * **Vandenberg’s Rule:** Simple diffusion is directly proportional to surface area and concentration gradient, but inversely proportional to membrane thickness (Fick’s Law).

Question 126: Skeletal muscle contraction ends when?

A. Ions move out of the cytoplasm
B. Acetylcholine is absorbed from the neuromuscular junction
C. Closure and indrawing of receptors
D. Calcium is pumped back into the sarcoplasmic reticulum (Correct Answer)

Explanation: **Explanation:** Skeletal muscle contraction is a calcium-dependent process. The correct answer is **D** because the relaxation phase begins when the stimulus from the motor neuron ceases, leading to the closure of Ryanodine receptors. The **SERCA pump (Sarcoplasmic/Endoplasmic Reticulum Calcium ATPase)** then actively transports $Ca^{2+}$ ions from the sarcoplasm back into the sarcoplasmic reticulum against a concentration gradient. As cytosolic calcium levels drop, calcium dissociates from **Troponin C**, allowing the Troponin-Tropomyosin complex to re-cover the active sites on actin, thereby preventing cross-bridge cycling. **Analysis of Incorrect Options:** * **Option A:** While ions (like $K^+$) move out of the cell during repolarization to reset the membrane potential, this does not directly terminate the mechanical contraction; the sequestration of calcium is the definitive "off switch." * **Option B:** Acetylcholine (ACh) is not "absorbed"; it is rapidly **hydrolyzed** by the enzyme Acetylcholinesterase (AChE) into choline and acetate. * **Option C:** Receptors do not "indraw" to end contraction. While prolonged exposure to ACh can lead to receptor desensitization, it is not the physiological mechanism for ending a single twitch. **High-Yield NEET-PG Pearls:** * **Calsequestrin:** A protein within the sarcoplasmic reticulum that binds to $Ca^{2+}$, allowing it to be stored at high concentrations. * **Malignant Hyperthermia:** Caused by a mutation in the **Ryanodine receptor (RyR1)**, leading to excessive calcium release and sustained muscle contraction/heat production. * **Rigor Mortis:** Occurs because **ATP is required for the relaxation phase** (to power the SERCA pump and to detach the myosin head from actin). Without ATP, the cross-bridges remain locked.

Question 127: Calcium is pumped back into the sarcoplasmic reticulum by which mechanism?

A. SERCA (Sarco/Endoplasmic Reticulum Calcium ATPase) (Correct Answer)
B. Sodium-potassium pump
C. Ryanodine receptor
D. None of the above

Explanation: **Explanation:** The correct answer is **SERCA (Sarco/Endoplasmic Reticulum Calcium ATPase)**. **1. Why SERCA is correct:** Muscle relaxation is an active process that requires the removal of calcium ions ($Ca^{2+}$) from the sarcoplasm back into the Sarcoplasmic Reticulum (SR). This occurs against a steep concentration gradient, necessitating **Primary Active Transport**. The SERCA pump utilizes ATP hydrolysis to transport two $Ca^{2+}$ ions into the SR for every ATP molecule consumed. This decrease in cytosolic calcium allows $Ca^{2+}$ to dissociate from Troponin C, leading to the cessation of cross-bridge cycling and subsequent muscle relaxation. **2. Why other options are incorrect:** * **Sodium-potassium pump ($Na^+/K^+$ ATPase):** This is a primary active transporter found in almost all cell membranes. It maintains the resting membrane potential by pumping 3 $Na^+$ out and 2 $K^+$ in; it does not directly transport calcium into the SR. * **Ryanodine receptor (RyR):** These are calcium-release channels located on the SR membrane. They are responsible for the efflux of calcium *out* of the SR into the cytoplasm during excitation-contraction coupling, not its reuptake. **3. High-Yield Clinical Pearls for NEET-PG:** * **Phospholamban:** In cardiac muscle, SERCA is regulated by a protein called phospholamban. When dephosphorylated, it inhibits SERCA; when phosphorylated (via $\beta$-adrenergic stimulation), inhibition is lifted, increasing the rate of relaxation (**Lusitropic effect**). * **Calsequestrin:** Inside the SR, calcium binds to this protein, which allows the SR to store high concentrations of $Ca^{2+}$ without significantly increasing the gradient against which SERCA must pump. * **Malignant Hyperthermia:** Caused by a mutation in the Ryanodine receptor (RyR1), leading to excessive calcium release.

Question 128: What is the primary function of the Golgi tendon organ?

A. Detecting muscle length
B. Detecting reflex muscle tension (Correct Answer)
C. Monitoring nutritional status
D. Regulating excretory function

Explanation: ### Explanation The **Golgi Tendon Organ (GTO)** is a specialized sensory receptor located at the junction of muscle fibers and tendons. It is arranged **in series** with the muscle fibers, making it exquisitely sensitive to changes in **muscle tension**. **1. Why Option B is Correct:** When a muscle undergoes forceful contraction, the GTO is stretched. It sends impulses via **Ib afferent nerve fibers** to the spinal cord. These fibers synapse on inhibitory interneurons, which then inhibit the alpha motor neurons of the same muscle. This mechanism, known as the **Inverse Stretch Reflex (Autogenic Inhibition)**, prevents muscle damage by causing the muscle to relax when tension becomes excessive. **2. Why Other Options are Incorrect:** * **Option A:** Detecting muscle length is the primary function of the **Muscle Spindle**. Muscle spindles are arranged **in parallel** with extrafusal fibers and mediate the stretch reflex (e.g., knee jerk). * **Option C & D:** Nutritional status and excretory functions are regulated by metabolic, endocrine, and autonomic systems (e.g., hypothalamus, kidneys), not by musculoskeletal mechanoreceptors. **3. High-Yield Clinical Pearls for NEET-PG:** * **Arrangement:** Muscle Spindle = Parallel; GTO = Series. * **Afferent Fibers:** Muscle Spindle = **Ia** (primary) and **II** (secondary); GTO = **Ib**. * **The "Clasp-Knife Response":** In upper motor neuron (UMN) lesions, exaggerated GTO activity contributes to the sudden "melting away" of resistance when a limb is passively stretched, a classic clinical sign of spasticity. * **Function:** While the muscle spindle is a feedback regulator of muscle **length**, the GTO is a feedback regulator of muscle **force/tension**.

Question 129: Which of the following statements about ion composition in the body is true?

A. Intracellular and extracellular ion compositions are the same.
B. Phosphorus and magnesium are major ions intracellularly.
C. Sodium and chloride are the principal ions in extracellular fluid. (Correct Answer)
D. The kidney tightly regulates sodium, potassium, and chloride composition.

Explanation: ### Explanation **1. Why Option C is Correct:** The body’s fluid compartments are divided into **Extracellular Fluid (ECF)** and **Intracellular Fluid (ICF)**. The ECF (which includes plasma and interstitial fluid) is characterized by high concentrations of **Sodium (Na⁺)** and **Chloride (Cl⁻)**. Sodium is the primary osmotically active cation in the ECF, responsible for maintaining fluid volume and osmolarity. Chloride serves as the major balancing anion. **2. Why the Other Options are Incorrect:** * **Option A:** This is false. There is a marked **chemical disequilibrium** between the ICF and ECF maintained by the Na⁺-K⁺ ATPase pump. For example, Na⁺ is high outside, while K⁺ is high inside. * **Option B:** While Magnesium and Phosphorus are indeed found intracellularly, the **major** intracellular ions are **Potassium (K⁺)** (the primary cation) and **Proteins/Organic Phosphates** (the primary anions). * **Option D:** While the kidney is the primary organ for regulating these ions, the statement is technically less specific than Option C in the context of "ion composition." Furthermore, the kidney regulates *excretion* to maintain *homeostasis*, but the *composition* itself is a fundamental property of the biological compartments. (In multiple-choice exams, the most direct physiological fact—Option C—is the preferred answer). **3. High-Yield Clinical Pearls for NEET-PG:** * **Major Cations:** ECF = Na⁺ (~142 mEq/L); ICF = K⁺ (~140 mEq/L). * **Major Anions:** ECF = Cl⁻ and HCO₃⁻; ICF = Phosphates and Proteins. * **Gibbs-Donnan Effect:** Explains why plasma has a slightly higher protein concentration and different ion distribution compared to interstitial fluid. * **Osmolarity:** Despite different ion compositions, the **total osmolarity** of the ICF and ECF is equal (~290–300 mOsm/L) because water moves freely to maintain osmotic equilibrium.

Question 130: What is the term for the length of a muscle fiber at the start of contraction that is necessary to achieve maximum active tension?

A. Equilibrium length
B. Resting length (Correct Answer)
C. Initial length
D. Overlapping length

Explanation: **Explanation:** The correct answer is **Resting length (Option B)**. This concept is fundamental to the **Length-Tension Relationship** in muscle physiology. **1. Why Resting Length is Correct:** The resting length ($L_0$) is defined as the optimal length of a muscle fiber at which it can develop the maximum active tension during contraction. At this specific length, there is **maximal overlap** between the actin (thin) and myosin (thick) filaments. This allows for the greatest number of cross-bridge formations. If the muscle is stretched beyond or shortened below this length, the number of potential cross-bridges decreases, thereby reducing the active tension produced. **2. Why the Other Options are Incorrect:** * **Equilibrium length (A):** This refers to the length of a relaxed muscle when it is not attached to the skeleton. At this length, the passive tension is zero, but it is typically shorter than the optimal resting length. * **Initial length (C):** While "initial length" refers to the length before contraction, it is a general term. It does not specifically denote the *optimal* length required for *maximum* tension. * **Overlapping length (D):** This is a descriptive term for the zone where filaments meet, but it is not the formal physiological term for the fiber length itself. **3. NEET-PG High-Yield Pearls:** * **Frank-Starling Law:** This is the cardiac application of the length-tension relationship; increased initial stretching of cardiac myocytes (preload) leads to increased force of contraction, up to a physiological limit. * **Sarcomere Length:** In mammalian skeletal muscle, the optimal sarcomere length for maximum tension is approximately **2.0 to 2.2 μm**. * **Total Tension:** Remember that Total Tension = Active Tension (from cross-bridges) + Passive Tension (from elastic elements like titin). Maximum *active* tension occurs specifically at $L_0$.

Practice by Chapter

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