General Physiology — MCQs

General Physiology Indian Medical PG Practice Questions and MCQs

Question 331: Activation of G-protein regulates all of the following except?

A. Adenyl cyclase activity
B. Ion channels
C. IP3 & DAG
D. Transcription factor (Correct Answer)

Explanation: **Explanation:** G-protein coupled receptors (GPCRs) are the largest family of cell surface receptors. They act as molecular switches that transmit signals from extracellular ligands to intracellular effectors via heterotrimeric G-proteins ($\alpha$, $\beta$, and $\gamma$ subunits). **Why "Transcription Factor" is the correct answer:** Transcription factors are typically regulated by **nuclear receptors** (e.g., steroid hormones) or via complex downstream kinase cascades (like the MAPK pathway). While G-protein signaling can *eventually* influence gene expression, G-proteins do not **directly** regulate transcription factors. Transcription factors are located in the nucleus, whereas G-proteins are membrane-bound peripheral proteins. **Analysis of Incorrect Options:** * **Adenyl cyclase activity:** This is a classic G-protein function. $G_s$ stimulates and $G_i$ inhibits Adenyl Cyclase, which converts ATP to cAMP (the second messenger). * **Ion channels:** G-proteins can regulate ion channels either directly (e.g., $G_{\beta\gamma}$ subunits opening $K^+$ channels in the heart) or indirectly via second messengers (e.g., cAMP-dependent phosphorylation of $Ca^{2+}$ channels). * **IP3 & DAG:** The $G_q$ protein activates Phospholipase C (PLC), which cleaves membrane phospholipids into Inositol triphosphate (IP3) and Diacylglycerol (DAG), leading to calcium release and Protein Kinase C activation. **High-Yield Clinical Pearls for NEET-PG:** * **Toxins:** *Vibrio cholerae* toxin causes permanent activation of $G_s$ (inhibits GTPase activity), leading to secretory diarrhea. *Bordetella pertussis* toxin inhibits $G_i$, leading to increased cAMP levels. * **Structure:** GPCRs are also known as **7-transmembrane** or **Serpentine receptors**. * **Termination:** The signal is terminated when the $\alpha$-subunit hydrolyzes GTP to GDP (intrinsic GTPase activity).

Question 332: In peripheral tissues, which of the following contains substance P?

A. Plasma cell
B. Mast cell
C. Nerve terminal (Correct Answer)
D. Vascular endothelium

Explanation: **Explanation:** **Substance P** is an 11-amino acid neuropeptide belonging to the tachykinin family. It is primarily synthesized in the cell bodies of **primary afferent (sensory) neurons** located in the dorsal root ganglia. From there, it is transported to both the central terminals (in the spinal cord) and the **peripheral nerve terminals** (nociceptors). 1. **Why Nerve Terminal is Correct:** In the periphery, Substance P is stored in the vesicles of **unmyelinated C-fibers**. Upon stimulation (injury or inflammation), it is released from these terminals. It functions as a potent vasodilator and increases vascular permeability, contributing to **neurogenic inflammation**. 2. **Why Other Options are Incorrect:** * **Plasma Cells:** These are differentiated B-cells responsible for antibody (immunoglobulin) production, not neuropeptide storage. * **Mast Cells:** While mast cells are *activated* by Substance P (causing degranulation and histamine release), they do not synthesize or contain Substance P themselves. * **Vascular Endothelium:** The endothelium responds to Substance P via NK1 receptors to release nitric oxide, but it is not a source of the peptide. **High-Yield Clinical Pearls for NEET-PG:** * **Receptor:** Substance P acts primarily via the **NK1 (Neurokinin-1) receptor**. * **Triple Response of Lewis:** Substance P release from peripheral nerve terminals is a key mediator of the "flare" component in the triple response. * **Pain Transmission:** In the CNS, it is a major neurotransmitter for pain signals in the **Substantia Gelatinosa** of the spinal cord. * **Pharmacology:** **Aprepitant** is an NK1 receptor antagonist used clinically to manage chemotherapy-induced nausea and vomiting (CINV).

Question 333: What is the clearance of a drug?

A. The volume of plasma cleared of drug per unit of time. (Correct Answer)
B. The amount of drug excreted in urine.
C. The amount of drug metabolized per unit of time.
D. All of the above.

Explanation: **Explanation** Clearance ($Cl$) is a fundamental pharmacokinetic parameter defined as the **volume of plasma** from which a drug is completely removed per unit of time (usually expressed in mL/min or L/hr). It represents the efficiency of drug elimination from the body. The mathematical formula is: $Cl = \frac{\text{Rate of elimination}}{\text{Plasma concentration } (C_p)}$ **Why Option A is correct:** Clearance does not measure the *amount* of drug removed; rather, it measures the *volume of fluid* (plasma) that would need to be entirely freed of the drug to account for the elimination. This is a constant value for drugs following first-order kinetics. **Why other options are incorrect:** * **Option B:** The amount of drug in urine is the "excretion rate," not clearance. Clearance relates this excretion to the plasma concentration. * **Option C:** This describes the "metabolic rate." While metabolism contributes to total clearance (Hepatic Clearance), it is only one component and is measured in mass/time (e.g., mg/min), not volume/time. * **Option D:** Incorrect because clearance specifically refers to the volume of distribution cleared, not the mass of the drug. **NEET-PG High-Yield Pearls:** 1. **Total Body Clearance:** Sum of Renal + Hepatic + Pulmonic + Other clearances. 2. **First-Order Kinetics:** Clearance remains constant regardless of drug concentration. 3. **Zero-Order Kinetics:** Clearance decreases as plasma concentration increases (e.g., Phenytoin, Alcohol, Aspirin at high doses). 4. **Clinical Significance:** Clearance is the most important parameter for determining the **Maintenance Dose** of a drug. * *Maintenance Dose = Target $C_p \times Cl$*

Question 334: The transmembrane region of a protein is likely to have which of the following?

A. A stretch of hydrophilic amino acids
B. A stretch of hydrophobic amino acids (Correct Answer)
C. A disulphide loop
D. Alternating hydrophilic and hydrophobic amino acids

Explanation: **Explanation:** The cell membrane is a **lipid bilayer** consisting of phospholipids with polar (hydrophilic) heads facing the aqueous environment and non-polar (**hydrophobic**) fatty acid tails forming the interior core. For a protein to span this membrane (transmembrane protein), the portion embedded within the bilayer must be compatible with this oily, non-polar environment. **Why Option B is Correct:** The transmembrane region typically consists of a stretch of **20–25 hydrophobic amino acids** (such as Valine, Leucine, Isoleucine, and Phenylalanine). These amino acids often arrange themselves into an **alpha-helix** configuration. This structure allows the hydrophobic side chains to interact with the fatty acid tails of the phospholipids via van der Waals forces, anchoring the protein within the membrane. **Why Other Options are Incorrect:** * **Option A:** Hydrophilic amino acids are water-loving. If placed in the lipid core, they would be energetically unstable. They are instead found in the extracellular and intracellular domains of the protein. * **Option C:** Disulphide loops (cysteine bonds) are primarily found in the extracellular domains of proteins or secreted proteins to stabilize their 3D structure; they do not define the transmembrane segment. * **Option D:** Alternating sequences are characteristic of certain beta-sheets (like porins), but the standard "transmembrane region" referred to in general physiology is the hydrophobic alpha-helix. **High-Yield Clinical Pearls for NEET-PG:** * **Hydropathy Plot:** A graphical tool used to predict transmembrane segments by identifying long stretches of hydrophobic amino acids. * **G-Protein Coupled Receptors (GPCRs):** These are the most common transmembrane proteins, characterized by **seven** hydrophobic spanning segments (7-Pass receptors). * **Stop-Transfer Sequence:** The specific hydrophobic sequence that signals the ribosome to stop translocation into the ER lumen, ensuring the protein remains embedded in the membrane.

Question 335: All of the following are associated with increased aging, except?

A. Enlargement of telomere (Correct Answer)
B. Decrease in number of mitochondria
C. Glycosylation of DNA
D. Glycosylation of RNA

Explanation: **Explanation:** The correct answer is **A. Enlargement of telomere**. This is because aging is characterized by the progressive **shortening of telomeres**, not their enlargement. **1. Why Option A is correct (The Concept of Telomere Attrition):** Telomeres are repetitive nucleotide sequences (TTAGGG) at the ends of chromosomes that protect them from deterioration. According to the **Hayflick Limit**, somatic cells have a finite capacity for division. With each cell cycle, a small portion of telomeric DNA is lost because DNA polymerase cannot fully replicate the 3' end of linear chromosomes. When telomeres reach a critically short length, the cell enters **senescence** (permanent cell cycle arrest). Therefore, shortened telomeres are a hallmark of cellular aging. **2. Analysis of Incorrect Options:** * **B. Decrease in number of mitochondria:** Aging is associated with "mitochondrial dysfunction." There is a documented decline in mitochondrial biogenesis and an increase in mitochondrial DNA (mtDNA) mutations, leading to a reduced number of functional mitochondria and decreased ATP production. * **C & D. Glycosylation of DNA and RNA:** Aging involves the non-enzymatic attachment of sugars to macromolecules (proteins and nucleic acids), forming **Advanced Glycation End-products (AGEs)**. This process causes cross-linking, structural damage, and functional impairment of DNA and RNA, contributing to the aging phenotype. **Clinical Pearls for NEET-PG:** * **Telomerase:** An enzyme (ribonucleoprotein) that can add telomeric repeats. It is active in germ cells and **cancer cells** (providing them immortality) but is absent or low in most somatic cells. * **Werner Syndrome:** A progeroid syndrome (premature aging) caused by a mutation in the *WRN* gene, which codes for a DNA helicase involved in telomere maintenance. * **Free Radical Theory:** Aging is also driven by Reactive Oxygen Species (ROS) causing oxidative damage to lipids and proteins.

Question 336: What is hypertrophy?

A. Increase in cell number
B. Increase in cell size (Correct Answer)
C. Decrease in cell number
D. Decrease in cell size

Explanation: **Explanation:** **Hypertrophy** is defined as an increase in the size of cells, which subsequently leads to an increase in the size of the involved organ. This process occurs due to the increased synthesis of structural proteins and organelles within the cell. It is the primary response to increased workload in cells that have a limited capacity to divide (permanent cells). **Analysis of Options:** * **Option B (Correct):** Hypertrophy specifically refers to **cellular enlargement**. A classic physiological example is the skeletal muscle growth seen in bodybuilders, while a pathological example is Left Ventricular Hypertrophy (LVH) due to hypertension. * **Option A (Incorrect):** An increase in the number of cells is termed **Hyperplasia**. This occurs in tissues capable of replication (e.g., uterine growth during pregnancy involves both hypertrophy and hyperplasia). * **Option C & D (Incorrect):** A decrease in cell number or size is characteristic of **Atrophy**, which results in the shrinkage of an organ or tissue. **High-Yield NEET-PG Pearls:** 1. **Permanent Cells:** Cardiac myocytes and skeletal muscle cells undergo *only* hypertrophy, not hyperplasia, because they cannot divide. 2. **Mechanism:** Triggered by mechanical sensors (stretch), growth factors (IGF-1), and vasoactive agents (Endothelin-1, Angiotensin II). 3. **Subcellular Hypertrophy:** An example is the hypertrophy of the Smooth Endoplasmic Reticulum (SER) in hepatocytes when exposed to drugs like barbiturates (leading to drug tolerance). 4. **Key Distinction:** Hypertrophy (Size) vs. Hyperplasia (Number). Both result in increased organ volume.

Question 337: What does the term "Meileu interior" refer to in physiology?

A. Extracellular fluid (ECF) (Correct Answer)
B. Intracellular fluid (ICF)
C. Plasma
D. Lymph

Explanation: **Explanation:** The term **"Milieu intérieur"** (French for "the environment within") was coined by the 19th-century French physiologist **Claude Bernard**. It refers specifically to the **Extracellular Fluid (ECF)** that surrounds and bathes the cells of multicellular organisms. **Why ECF is the correct answer:** Claude Bernard proposed that for an organism to remain healthy and independent of the external environment, its internal environment must remain constant. Since cells do not come into direct contact with the outside world, they rely on the ECF (interstitial fluid and plasma) for nutrients and waste removal. This concept laid the foundation for **Homeostasis**, a term later popularized by Walter Cannon. **Analysis of Incorrect Options:** * **Intracellular Fluid (ICF):** This is the fluid *inside* the cells. While vital, it is not the "internal environment" described by Bernard; rather, it is the environment that the *Milieu intérieur* protects and maintains. * **Plasma:** Plasma is a sub-component of the ECF (along with interstitial fluid). While it is part of the internal environment, "Milieu intérieur" encompasses the *entire* ECF. * **Lymph:** Similar to plasma, lymph is merely a specialized part of the ECF. **NEET-PG High-Yield Pearls:** * **Claude Bernard:** Known as the "Father of Modern Physiology"; he introduced the concept of *Milieu intérieur*. * **Walter Cannon:** Coined the term **Homeostasis** to describe the maintenance of a constant *Milieu intérieur*. * **ECF Composition:** ECF constitutes approximately **20%** of total body weight (15% interstitial fluid, 5% plasma). * **Total Body Water (TBW):** Roughly 60% of body weight; ICF makes up 40%, and ECF makes up 20%.

Question 338: Hypoxia causes vasoconstriction in which of the following locations?

A. Muscle
B. Lungs (Correct Answer)
C. Liver
D. Spleen

Explanation: ### Explanation The correct answer is **Lungs (Option B)**. **Underlying Concept: Hypoxic Pulmonary Vasoconstriction (HPV)** In most systemic tissues, hypoxia (low oxygen) acts as a potent **vasodilator** to increase blood flow and oxygen delivery to oxygen-starved cells. However, the pulmonary circulation exhibits a unique phenomenon known as **Hypoxic Pulmonary Vasoconstriction (HPV)**. When alveolar oxygen levels drop, the smooth muscles of the pulmonary arterioles constrict. This is a protective mechanism designed to **shunt blood away** from poorly ventilated areas of the lung toward well-ventilated areas, thereby optimizing **ventilation-perfusion (V/Q) matching** and preventing systemic hypoxemia. **Why the other options are incorrect:** * **Muscle (A):** In skeletal muscle, hypoxia and the accumulation of metabolic byproducts (like adenosine, $K^+$, and $CO_2$) cause **vasodilation** to meet the increased metabolic demand. * **Liver (C) and Spleen (D):** These systemic organs follow the general rule where hypoxia leads to vasodilation to maintain tissue oxygenation. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** HPV is mediated by the inhibition of oxygen-sensitive $K^+$ channels in pulmonary artery smooth muscle cells, leading to depolarization and $Ca^{2+}$ influx. * **Clinical Correlation:** Chronic hypoxia (e.g., in COPD or at high altitudes) leads to generalized pulmonary vasoconstriction, resulting in **Pulmonary Hypertension** and eventually **Right Heart Failure (Cor Pulmonale)**. * **Fetal Circulation:** In the fetus, the lungs are hypoxic and collapsed, leading to high pulmonary vascular resistance, which shunts blood through the ductus arteriosus.

Question 339: What is the twitch of a single motor unit called?

A. Myoclonic jerk
B. Fasciculation (Correct Answer)
C. Tremor
D. Fibrillation

Explanation: **Explanation:** The correct answer is **Fasciculation**. **1. Why Fasciculation is correct:** A **motor unit** consists of a single alpha motor neuron and all the muscle fibers it innervates. A **fasciculation** is defined as the spontaneous, involuntary contraction of all muscle fibers within a single motor unit. Because an entire motor unit is involved, these twitches are often visible under the skin as brief ripples or flickers, though they are usually insufficient to move a joint. They typically result from pathological irritation of the lower motor neuron (LMN) or its axon. **2. Why the other options are incorrect:** * **Myoclonic jerk:** These are sudden, brief, shock-like involuntary movements caused by muscular contractions (positive myoclonus) or sudden loss of muscle tone (negative myoclonus). Unlike fasciculations, these involve large groups of muscles and result in visible limb movement. * **Tremor:** This is a rhythmic, oscillatory movement produced by alternating or synchronous contractions of antagonist muscles. It is not a single twitch but a repetitive movement. * **Fibrillation:** This is the spontaneous contraction of a **single muscle fiber**. Because only one fiber is contracting, fibrillations are **not visible** to the naked eye and can only be detected via Electromyography (EMG). They occur when a muscle fiber loses its nerve supply (denervation hypersensitivity). **3. High-Yield Clinical Pearls for NEET-PG:** * **Fasciculations vs. Fibrillations:** Fasciculations are visible; Fibrillations are invisible (EMG only). * **LMN Lesions:** Both fasciculations and fibrillations are classic signs of Lower Motor Neuron (LMN) lesions (e.g., Amyotrophic Lateral Sclerosis or Polio). * **Benign Fasciculations:** These can occur in healthy individuals due to fatigue, caffeine, or electrolyte imbalances (e.g., eyelid twitching).

Question 340: What is the primary use of kinesiomyography?

A. Monitoring of exercise capacity
B. Monitoring of neuromuscular function (Correct Answer)
C. Monitoring of muscle spindle activity
D. Monitoring of cerebrospinal fluid pressure changes

Explanation: **Explanation:** **Kinesiomyography (KMG)** is a method used for the quantitative monitoring of **neuromuscular function**, particularly during general anesthesia. It operates on the principle of **mechanomyography**, using a piezo-electric sensor (accelerometer) to measure the mechanical movement or displacement of a muscle (usually the adductor pollicis) in response to a nerve stimulus. This allows clinicians to objectively assess the degree of neuromuscular blockade and ensure safe recovery before extubation. **Analysis of Options:** * **Option A (Exercise capacity):** This is typically assessed using Cardiopulmonary Exercise Testing (CPET) or the 6-minute walk test, which measure VO2 max and metabolic parameters, not muscle displacement. * **Option C (Muscle spindle activity):** Muscle spindle activity (proprioception and stretch reflex) is evaluated via Electromyography (EMG) or H-reflex studies, which measure electrical potentials rather than mechanical motion. * **Option D (CSF pressure):** This is monitored via lumbar puncture manometry or intracranial pressure (ICP) transducers. **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard:** While KMG is common in clinical practice, **Acceleromyography (AMG)** is currently the most widely used objective monitor in anesthesia. * **Train-of-Four (TOF) Ratio:** In neuromuscular monitoring, a TOF ratio of **>0.9** is the clinical benchmark required to safely exclude residual paralysis. * **Site of Placement:** The most common site for monitoring is the **Ulnar nerve**, observing the contraction of the **Adductor pollicis** muscle. * **KMG vs. EMG:** EMG measures the *electrical* action potential of the muscle, whereas KMG/AMG measures the *mechanical* response.

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