General Physiology — MCQs

General Physiology Indian Medical PG Practice Questions and MCQs

Question 91: What are Watson and Crick most known for?

A. Discovering the helical structure of DNA (Correct Answer)
B. Associating Helicobacter Pylori with Chronic Gastritis
C. Discovering the HIV virus
D. None of the above

Explanation: **Explanation:** **Correct Option: A (Discovering the helical structure of DNA)** In 1953, James Watson and Francis Crick proposed the **Double Helix model** of DNA. Their discovery, based significantly on the X-ray diffraction data produced by Rosalind Franklin and Maurice Wilkins, revealed that DNA consists of two antiparallel polynucleotide chains held together by hydrogen bonds between complementary nitrogenous bases (Adenine-Thymine and Cytosine-Guanine). This discovery provided the molecular basis for genetic inheritance and earned them the Nobel Prize in 1962. **Incorrect Options:** * **Option B:** The association between *Helicobacter pylori* and chronic gastritis/peptic ulcer disease was discovered by **Barry Marshall and Robin Warren** (Nobel Prize 2005). * **Option C:** The HIV virus was discovered by **Luc Montagnier and Françoise Barré-Sinoussi** (Nobel Prize 2008). **NEET-PG High-Yield Facts:** * **DNA Structure:** The double helix is "right-handed" (B-DNA is the most common physiological form). * **Chargaff’s Rule:** In a double-stranded DNA molecule, the amount of Adenine equals Thymine (A=T) and Guanine equals Cytosine (G=C). * **Dimensions:** One complete turn of the B-DNA helix is **3.4 nm** (34 Å) long and contains **10 base pairs**, with a diameter of **2 nm**. * **Bonding:** A-T pairs have 2 hydrogen bonds; G-C pairs have 3 hydrogen bonds (making G-C rich regions more stable/harder to denature).

Question 92: Which is the most diffusible ion in excitable tissue?

A. Na+
B. K+ (Correct Answer)
C. Ca++
D. Cl-

Explanation: ### Explanation The correct answer is **B. K+ (Potassium)**. **1. Why K+ is the correct answer:** In a resting excitable cell (nerve or muscle), the Resting Membrane Potential (RMP) is primarily determined by the permeability of the cell membrane to specific ions. The cell membrane contains a high density of **"Leak Channels"** for Potassium. These channels are constitutively open, making the membrane **20 to 100 times more permeable to K+** than to Na+ or other ions at rest. Because K+ can move freely down its concentration gradient (from inside to outside), it is the most diffusible ion and the main determinant of the RMP (which sits close to the equilibrium potential of K+, approximately -94 mV). **2. Why the other options are incorrect:** * **Na+ (Sodium):** At rest, sodium channels are mostly closed. Na+ permeability is very low; it only becomes the most diffusible ion during the **depolarization** phase of an action potential when voltage-gated Na+ channels open. * **Ca++ (Calcium):** Calcium has very low membrane permeability at rest. Its movement is strictly regulated and usually occurs through specific voltage-gated or ligand-gated channels during signaling or muscle contraction. * **Cl- (Chloride):** While Cl- is more diffusible than Na+, its permeability is significantly lower than K+ in most neurons. Its movement often follows the electrical gradient established by K+. **3. High-Yield NEET-PG Pearls:** * **Goldman-Hodgkin-Katz Equation:** This formula calculates the RMP by considering the concentration gradients and the **relative permeability** of all ions. * **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 proteins. * **Clinical Correlation:** Changes in extracellular K+ (Hyperkalemia/Hypokalemia) have the most profound and immediate effect on RMP and cardiac excitability because K+ is the most diffusible ion.

Question 93: All the following factors stimulate growth hormone secretion except?

A. Deep sleep
B. Increased blood amino acids
C. Exercise
D. Increased blood glucose (Correct Answer)

Explanation: **Explanation:** Growth Hormone (GH) secretion is pulsatile and regulated by the hypothalamus via Growth Hormone-Releasing Hormone (GHRH) and Somatostatin (GHIH). Its release is primarily triggered by states of energy deficiency or increased metabolic demand. **Why Increased Blood Glucose is the Correct Answer:** Hyperglycemia (increased blood glucose) acts as a potent **inhibitor** of GH secretion. When blood glucose levels are high, the body suppresses GH to prevent further glucose elevation, as GH is a "diabetogenic" hormone that decreases peripheral glucose uptake and increases gluconeogenesis. Conversely, **hypoglycemia** is a powerful stimulator of GH. **Analysis of Incorrect Options:** * **Deep Sleep (Stage 3 & 4 NREM):** The largest peak of GH secretion occurs roughly 60 minutes after the onset of deep sleep. This is why adequate sleep is critical for growth in children. * **Increased Blood Amino Acids:** High levels of amino acids (especially Arginine) stimulate GH secretion. This promotes protein synthesis and tissue growth, utilizing the available building blocks. * **Exercise:** Physical stress and exercise are potent physiological stimuli for GH release, mediated by increased sympathetic activity and the need for fuel mobilization (lipolysis). **High-Yield Clinical Pearls for NEET-PG:** 1. **Gherlin:** Produced by the stomach, it is the only peripheral hormone that directly stimulates GH secretion (the "hunger hormone"). 2. **Somatomedins (IGF-1):** GH acts on the liver to produce IGF-1, which exerts negative feedback on GH secretion. 3. **L-Dopa & Clonidine:** Both are pharmacological stimulators used in clinical "GH stimulation tests" to diagnose deficiency. 4. **Obesity:** GH secretion is significantly **decreased** in obese individuals.

Question 94: Active tension in a muscle depends upon which of the following?

A. Number of muscle fibers
B. Number of motor units recruited (Correct Answer)
C. Aerobic capacity of muscle
D. Action potential amplitude

Explanation: ### Explanation **Correct Answer: B. Number of motor units recruited** **Mechanism:** Active tension is the force generated by the contraction of muscle proteins (actin and myosin cross-bridging) when a muscle is stimulated. According to the **Size Principle (Henneman's Principle)**, the total tension produced by a muscle is primarily determined by **Motor Unit Recruitment**. A motor unit consists of a single motor neuron and all the muscle fibers it innervates. By increasing the number of active motor units, the body increases the number of contracting muscle fibers, thereby increasing the total active tension. **Analysis of Incorrect Options:** * **A. Number of muscle fibers:** While the total number of fibers in a muscle dictates its maximum potential strength, the *active* tension at any given moment depends on how many of those fibers are currently being stimulated to contract. * **C. Aerobic capacity:** This determines the muscle's endurance and resistance to fatigue (metabolic efficiency), not the immediate generation of active tension. * **D. Action potential amplitude:** Action potentials in skeletal muscle follow the **All-or-None Law**. The amplitude of an action potential is constant; force is modulated by the *frequency* of action potentials (summation) and the *number* of units recruited, not the size of the electrical signal itself. **High-Yield NEET-PG Pearls:** * **Active vs. Passive Tension:** Active tension is maximal at the **resting length ($L_0$)** of the sarcomere (approx. 2.0–2.2 $\mu m$), where actin-myosin overlap is optimal. Passive tension is due to the elastic elements (like titin) and increases as the muscle is stretched. * **Treppe (Staircase Phenomenon):** An increase in tension observed when a muscle is stimulated repeatedly shortly after the relaxation phase, due to increased cytosolic $Ca^{2+}$. * **Tetanization:** When the frequency of stimulation is so high that no relaxation occurs, leading to maximal sustained contraction.

Question 95: The equilibrium potential for an ion is calculated using which equation?

A. Gibbs-Donnan equation
B. Nernst equation (Correct Answer)
C. Goldman equation
D. Fick's law

Explanation: The **Nernst Equation** is the fundamental formula used to calculate the **equilibrium potential** (also known as the Nernst potential) for a single, specific ion. It determines the electrical potential difference across a cell membrane that exactly balances the concentration gradient of that ion, resulting in no net movement. ### Why the Nernst Equation is Correct: The equation is expressed as: **$E = \frac{RT}{zF} \ln \frac{[Ion]_{out}}{[Ion]_{in}}$** (Simplified at body temperature: $E = \frac{61}{z} \log_{10} \frac{[Ion]_{out}}{[Ion]_{in}}$) It accounts for the valence ($z$) and the concentration gradient of a **single ion** to find the voltage at which that ion is in electrochemical equilibrium. ### Explanation of Incorrect Options: * **Goldman-Hodgkin-Katz (GHK) Equation:** Unlike Nernst, this calculates the **Resting Membrane Potential (RMP)** by considering the concentrations and **permeabilities** of multiple ions (Na⁺, K⁺, and Cl⁻) simultaneously. * **Gibbs-Donnan Equation:** Describes the behavior of charged particles near a semi-permeable membrane when one ion is non-diffusible (e.g., intracellular proteins). It explains the resulting unequal distribution of permeant ions. * **Fick’s Law:** Describes the **rate of diffusion** of a gas or solute across a membrane. It states that the flux is proportional to the concentration gradient, surface area, and membrane permeability. ### High-Yield Clinical Pearls for NEET-PG: * **K⁺ Equilibrium Potential:** Approximately **-94 mV**. Since the RMP of a typical neuron is -70 to -90 mV, K⁺ is the primary determinant of RMP. * **Na⁺ Equilibrium Potential:** Approximately **+61 mV**. * **Key Concept:** If the membrane permeability to an ion increases, the membrane potential moves **toward** the equilibrium potential of that specific ion (e.g., during the upstroke of an action potential, the potential moves toward +61 mV due to Na⁺ influx).

Question 96: Plasma drug monitoring is done for which type of drugs?

A. Drugs with a high safety margin
B. Drugs with a low safety margin (Correct Answer)
C. Drugs with a high therapeutic index
D. None of the above

Explanation: **Explanation:** **Therapeutic Drug Monitoring (TDM)** or plasma drug monitoring is the clinical practice of measuring drug concentrations in the blood to maintain a constant concentration within a specific **therapeutic window**. **Why the correct answer is right:** Plasma drug monitoring is essential for drugs with a **low safety margin** (also known as a **narrow therapeutic index**). In these drugs, the dose required for a therapeutic effect is very close to the dose that causes toxicity. Small fluctuations in plasma concentration—due to drug interactions, organ dysfunction (renal/hepatic), or genetic variability—can lead to either treatment failure or severe adverse effects. Monitoring ensures the drug remains above the Minimum Effective Concentration (MEC) but below the Minimum Toxic Concentration (MTC). **Why the incorrect options are wrong:** * **Options A & C:** Drugs with a **high safety margin** or a **high therapeutic index** (e.g., Penicillin, Paracetamol) have a vast gap between the effective dose and the toxic dose. For these drugs, the clinical response (e.g., relief of pain or fever) is easy to monitor, and the risk of accidental toxicity is low, making routine blood monitoring unnecessary and cost-ineffective. **High-Yield Clinical Pearls for NEET-PG:** * **Common drugs requiring TDM:** Lithium, Digoxin, Theophylline, Phenytoin, Aminoglycosides (Gentamicin), and Immunosuppressants (Cyclosporine). * **Therapeutic Index (TI) Formula:** $TI = TD_{50} / ED_{50}$ (Toxic Dose for 50% of population / Effective Dose for 50%). * **Exceptions:** TDM is **not** useful for "hit and run" drugs (e.g., Omeprazole, MAO inhibitors) where the effect lasts much longer than the plasma concentration.

Question 97: Thermogenin is found in which of the following tissues?

A. Brain
B. Adipose tissue (Brown fat) (Correct Answer)
C. Skin
D. All of the above

Explanation: ### Explanation **Correct Answer: B. Adipose tissue (Brown fat)** **Mechanism and Concept:** Thermogenin, also known as **Uncoupling Protein 1 (UCP1)**, is a protein found in the inner mitochondrial membrane of **brown adipose tissue (BAT)**. In normal oxidative phosphorylation, the proton gradient generated by the electron transport chain is used by ATP synthase to produce ATP. Thermogenin "uncouples" this process by allowing protons to leak back into the mitochondrial matrix without passing through ATP synthase. Instead of being stored as chemical energy (ATP), the energy is dissipated as **heat**. This process is known as **non-shivering thermogenesis**, which is vital for neonates to maintain body temperature. **Why other options are incorrect:** * **A. Brain:** While the brain is metabolically active and produces heat, it does not contain UCP1-mediated thermogenic pathways. It relies on glucose oxidation for ATP production to maintain neuronal function. * **C. Skin:** The skin acts as an insulator and a site for heat dissipation (via sweat and vasodilation), but it does not possess brown fat or thermogenin to generate heat internally. * **D. All of the above:** Thermogenin expression is tissue-specific and restricted to brown adipocytes. **NEET-PG High-Yield Pearls:** * **Location of Brown Fat:** In newborns, it is found in the interscapular region, axilla, and around deep organs (kidneys/adrenals). In adults, it persists mainly in the cervical, supraclavicular, and paravertebral regions. * **Appearance:** Brown fat is "brown" due to a high density of **mitochondria** and a rich **capillary supply**. * **Regulation:** Non-shivering thermogenesis is stimulated by the **Sympathetic Nervous System** via **$\beta_3$-adrenergic receptors**, which activate lipolysis and upregulate UCP1. * **Clinical Correlation:** Hibernating animals rely heavily on thermogenin to survive cold temperatures.

Question 98: All of the following statements about phagocytosis are true, except?

A. Amoeba and other unicellular protozoa make their living out of this process.
B. Phagocytosis is used to ingest particles < 0.5 microns in size. (Correct Answer)
C. Phagocytosis is used to ingest particles > 0.5 microns in size.
D. Digestion occurs within phagolysosomes.

Explanation: **Explanation:** Phagocytosis, often referred to as "cell eating," is a specific form of endocytosis used by specialized cells (like macrophages and neutrophils) to ingest large particulate matter. **1. Why Option B is the Correct Answer (The False Statement):** Phagocytosis is strictly reserved for the ingestion of **large particles**, typically defined as those **> 0.5 µm** in diameter. This includes bacteria, dead tissue cells, and mineral particles. In contrast, the ingestion of particles or fluid droplets **< 0.5 µm** occurs via **pinocytosis** ("cell drinking") or receptor-mediated endocytosis. Therefore, the statement that phagocytosis is used for particles < 0.5 µm is physiologically incorrect. **2. Analysis of Other Options:** * **Option A:** In evolutionary terms, phagocytosis originated as a means of nutrition. Unicellular organisms like **Amoeba** utilize this process to capture and digest food particles for survival. * **Option C:** This is a true statement. Phagocytosis is triggered by the binding of ligands to receptors on the phagocyte, leading to the engulfment of large particles (> 0.5 µm). * **Option D:** Once a particle is engulfed, it forms a **phagosome**. This vesicle fuses with a lysosome to form a **phagolysosome**, where hydrolytic enzymes and reactive oxygen species digest the ingested material. **High-Yield Clinical Pearls for NEET-PG:** * **Professional Phagocytes:** Neutrophils, Macrophages, and Dendritic cells. * **Opsonization:** The process where particles (like bacteria) are coated with antibodies (IgG) or complement (C3b) to enhance the efficiency of phagocytosis. * **Residual Bodies:** Indigestible materials that remain in the cytoplasm after lysosomal digestion (e.g., lipofuscin granules). * **Key Difference:** Pinocytosis is constitutive (occurring continuously), whereas phagocytosis is a **triggered process** requiring receptor activation.

Question 99: Resting membrane potential is maintained due to which ion?

A. K+ (Correct Answer)
B. Na+
C. Mg++
D. Cl-

Explanation: ### Explanation The **Resting Membrane Potential (RMP)** is the electrical potential difference across the cell membrane when the cell is at rest. It is primarily determined by the **selective permeability** of the cell membrane and the **concentration gradients** of ions. **Why K+ is the correct answer:** 1. **High Permeability:** At rest, the cell membrane is significantly more permeable to **Potassium (K+)** than to any other ion (about 50–100 times more than Na+). This is due to the presence of numerous **K+ leak channels** that remain open at rest. 2. **Efflux of K+:** K+ follows its chemical gradient and leaks out of the cell. As positive charges leave, the interior of the cell becomes electronegative. 3. **Nernst Potential:** The RMP of a typical neuron (approx. -70 to -90 mV) sits very close to the equilibrium potential of K+ (-94 mV), proving that K+ is the dominant ion in establishing this state. **Why other options are incorrect:** * **Na+:** The membrane has very low permeability to Sodium at rest. Na+ influx is responsible for **depolarization** during an action potential, not the maintenance of RMP. * **Mg++:** Magnesium is an intracellular cation that acts as a cofactor for enzymes (like Na+-K+ ATPase) but does not directly determine the RMP. * **Cl-:** While Chloride contributes to the RMP in some cells (like skeletal muscle), its influence is secondary to K+ in most excitable tissues. **High-Yield Facts for NEET-PG:** * **Goldman-Hodgkin-Katz Equation:** Used to calculate RMP by considering the permeability and concentration of all major ions (K+, Na+, Cl-). * **Na+-K+ ATPase Pump:** While K+ leakage *establishes* the RMP, the Na+-K+ ATPase pump *maintains* the ionic gradients (pumping 3 Na+ out and 2 K+ in). It is described as **electrogenic** because it contributes about -4 to -5 mV to the RMP. * **Clinical Correlation:** Alterations in extracellular K+ (Hyperkalemia/Hypokalemia) have the most profound effect on RMP, often leading to cardiac arrhythmias.

Question 100: Which one of the following receptors acts by decreasing cAMP concentration when stimulated by Norepinephrine?

A. Alpha-2 (Correct Answer)
B. Beta-1
C. Vasopressin-2
D. GnRH

Explanation: The correct answer is **Alpha-2 (A)**. ### **Mechanism of Action** Adrenergic receptors are G-protein coupled receptors (GPCRs). The **Alpha-2 ($\alpha_2$) receptor** is coupled to the **$G_i$ (inhibitory)** protein. When norepinephrine binds to $\alpha_2$, it inhibits the enzyme **adenylyl cyclase**, leading to a decrease in the conversion of ATP to **cyclic AMP (cAMP)**. This reduction in cAMP levels results in the inhibition of neurotransmitter release (presynaptic effect) and smooth muscle contraction in certain vascular beds. ### **Analysis of Incorrect Options** * **Beta-1 ($\beta_1$):** These receptors are coupled to **$G_s$ (stimulatory)** proteins. Stimulation by norepinephrine activates adenylyl cyclase, **increasing** cAMP levels, which leads to positive inotropy and chronotropy in the heart. * **Vasopressin-2 ($V_2$):** Located in the renal collecting ducts, $V_2$ receptors are also **$G_s$ coupled**. They increase cAMP to facilitate the insertion of Aquaporin-2 channels. * **GnRH:** Gonadotropin-releasing hormone receptors are coupled to the **$G_q$ pathway**, which activates Phospholipase C (PLC) to increase Inositol triphosphate ($IP_3$) and Diacylglycerol (DAG), rather than primarily affecting cAMP. ### **NEET-PG High-Yield Pearls** * **Mnemonic for Adrenergic Receptors:** * **$Q-I-S-S$**: $\alpha_1$ ($G_q$), $\alpha_2$ ($G_i$), $\beta_1$ ($G_s$), $\beta_2$ ($G_s$). * **Clinical Correlation:** **Clonidine** and **Methyldopa** are $\alpha_2$ agonists used to treat hypertension by decreasing sympathetic outflow from the CNS. * **$\alpha_2$ Locations:** Presynaptic nerve terminals (autoreceptors), pancreatic $\beta$-cells (decreases insulin), and platelets (increases aggregation).

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