General Physiology — MCQs

General Physiology Indian Medical PG Practice Questions and MCQs

Question 141: Calmodulin acts by:

A. Opening of Na+ channels
B. Phosphorylation of myosin by protein kinase (Correct Answer)
C. Activation of the Na+-K+ pump
D. Dephosphorylation by protein kinase

Explanation: **Explanation:** **Mechanism of Action (Why Option B is correct):** Calmodulin is a calcium-binding messenger protein found in all eukaryotic cells. In smooth muscle contraction, it plays a pivotal role because smooth muscle lacks the troponin complex found in skeletal muscle. When intracellular $Ca^{2+}$ levels rise, four $Ca^{2+}$ ions bind to **Calmodulin**, forming a $Ca^{2+}$-Calmodulin complex. This complex activates an enzyme called **Myosin Light Chain Kinase (MLCK)**. MLCK then **phosphorylates** the regulatory light chain of the myosin head. This phosphorylation increases myosin ATPase activity, allowing the myosin head to bind to actin and initiate the cross-bridge cycle (contraction). **Why other options are incorrect:** * **Option A:** Sodium channel opening is typically mediated by voltage changes (action potentials) or ligand binding (e.g., Acetylcholine), not by Calmodulin. * **Option C:** The $Na^{+}$-$K^{+}$ pump is primarily regulated by ATP availability and specific hormones (like insulin or aldosterone), not directly by the Calmodulin-MLCK pathway. * **Option D:** Dephosphorylation of myosin leads to **relaxation**, not contraction. This process is mediated by **Myosin Light Chain Phosphatase (MLCP)**, which removes the phosphate group that MLCK added. **High-Yield Clinical Pearls for NEET-PG:** * **Stoichiometry:** 1 Calmodulin binds to **4 Calcium ions**. * **Smooth Muscle Relaxation:** Nitric Oxide (NO) causes relaxation by increasing cGMP, which inhibits MLCK and activates MLCP. * **Troponin vs. Calmodulin:** Remember the mnemonic: **S**keletal muscle uses **S**troke/Troponin; **S**mooth muscle uses **S**econd messenger/Calmodulin. * **Other functions:** Calmodulin also regulates the activation of phosphorylase kinase in glycogenolysis and modulates cyclic nucleotide phosphodiesterase.

Question 142: Which of the following is a compensatory mechanism in a patient with hypovolemic shock?

A. Increased renal blood flow
B. Decrease in cortisol
C. Decrease in vasopressin
D. Decreased cutaneous blood flow (Correct Answer)

Explanation: **Explanation:** In hypovolemic shock, the primary physiological goal is to maintain perfusion to vital organs (brain and heart) despite a drop in cardiac output. This is achieved through the activation of the **sympathetic nervous system** and the **Renin-Angiotensin-Aldosterone System (RAAS)**. **Why Option D is Correct:** The sympathetic surge causes widespread peripheral vasoconstriction via **α1-adrenergic receptors**. Blood is shunted away from "non-essential" vascular beds, such as the skin and skeletal muscle, to prioritize the cerebral and coronary circulation. This **decreased cutaneous blood flow** manifests clinically as cold, clammy skin—a hallmark sign of shock. **Why the other options are incorrect:** * **A. Increased renal blood flow:** In shock, renal blood flow **decreases** due to vasoconstriction of the afferent arterioles (mediated by sympathetic nerves and Angiotensin II). This leads to oliguria, a key clinical indicator of the severity of shock. * **B. Decrease in cortisol:** Stress triggers the hypothalamic-pituitary-adrenal (HPA) axis, leading to an **increase** in cortisol levels to help maintain vascular tone and metabolic homeostasis. * **C. Decrease in vasopressin:** Low blood pressure and reduced atrial stretch trigger a significant **increase** in Vasopressin (ADH) release from the posterior pituitary to promote water retention and peripheral vasoconstriction (via V1 receptors). **NEET-PG High-Yield Pearls:** * **The "Golden Rule" of Shock:** Vital organs (Brain/Heart) have poor α1-receptor density and exhibit **autoregulation**, allowing them to maintain flow while the skin and kidneys suffer ischemia. * **Baroreceptor Reflex:** This is the earliest compensatory mechanism, leading to tachycardia and increased systemic vascular resistance (SVR). * **Reverse Stress Relaxation:** A delayed compensatory mechanism where blood vessels gradually constrict around the remaining volume.

Question 143: Which of the following are molecular motors?

A. Kinesin
B. Dynein
C. Myosin
D. All of the above (Correct Answer)

Explanation: **Explanation:** Molecular motors are specialized proteins that convert chemical energy (ATP) into mechanical work to facilitate intracellular transport and movement. They move along cytoskeletal tracks—either microtubules or actin filaments. * **Kinesin:** These are microtubule-based motors that typically move toward the **plus-end** of the microtubule (anterograde transport). They are essential for moving vesicles and organelles from the cell body toward the periphery (e.g., axonal transport). * **Dynein:** These are also microtubule-based motors but move toward the **minus-end** (retrograde transport). Cytoplasmic dyneins carry cargo toward the cell center, while axonemal dyneins are responsible for the beating of cilia and flagella. * **Myosin:** These are **actin-based** motors. Myosin II is the primary motor involved in muscle contraction, while other isoforms (like Myosin V) are involved in short-range vesicle transport along actin filaments. Since all three proteins function as biological motors using ATP hydrolysis, **Option D** is the correct answer. **High-Yield Clinical Pearls for NEET-PG:** * **Kartagener Syndrome:** Caused by a defect in **Axonemal Dynein**, leading to immobile cilia, bronchiectasis, and situs inversus. * **Directionality:** Remember **"K"**inesin moves **"K"**ick-away (Anterograde/Plus-end) and **"D"**ynein moves **"D"**rag-in (Retrograde/Minus-end). * **Fast Axonal Transport:** Occurs at a rate of 400 mm/day and utilizes both Kinesin and Dynein. * **Viral Transport:** Certain viruses like Herpes and Rabies exploit **Dynein** for retrograde transport to reach the CNS.

Question 144: What do tyrosine kinase receptors mediate?

A. They have constitutively active tyrosine kinase domains.
B. They phosphorylate and activate ras directly.
C. They mediate cellular processes involved in growth and differentiation. (Correct Answer)
D. They are not phosphorylated upon activation.

Explanation: **Explanation:** **Receptor Tyrosine Kinases (RTKs)** are a major class of enzyme-linked receptors that play a pivotal role in signal transduction. **Why Option C is Correct:** RTKs are the primary receptors for most **growth factors** (e.g., Insulin, EGF, PDGF, IGF-1, VEGF). Upon ligand binding, they trigger signaling cascades (like the MAP kinase pathway) that regulate gene expression, ultimately controlling **cell survival, growth, proliferation, and differentiation**. This makes them central to both normal development and oncogenesis. **Analysis of Incorrect Options:** * **Option A:** RTKs do **not** have constitutively active domains. They are inactive monomers that only become active (via dimerization and cross-phosphorylation) after a specific ligand binds to the extracellular domain. * **Option B:** RTKs do not activate **Ras** directly. Instead, they use "adapter proteins" (like GRB2 and SOS). SOS acts as a Guanine Nucleotide Exchange Factor (GEF) that then activates Ras by swapping GDP for GTP. * **Option D:** Autophosphorylation is a hallmark of RTK activation. Once the receptor dimerizes, the kinase domains phosphorylate specific **tyrosine residues** on the cytoplasmic tail of the opposite receptor strand. **NEET-PG High-Yield Pearls:** * **The "Insulin Exception":** Most RTKs are monomers that dimerize upon ligand binding. However, the **Insulin receptor** is a pre-formed heterotetramer ($\alpha_2\beta_2$). * **Clinical Link:** Mutations leading to *constitutive* activation of RTKs are a common cause of cancer (e.g., the *HER2/neu* receptor in breast cancer). * **JAK-STAT vs. RTK:** Do not confuse RTKs with Cytokine receptors (e.g., GH, Prolactin). RTKs have **intrinsic** kinase activity, while Cytokine receptors lack it and must recruit **JAK** (Janus Kinase) to signal.

Question 145: What type of cell junction is found in smooth muscle?

A. Zonula adherens
B. Macula adherens
C. Tight junction
D. Gap junction (Correct Answer)

Explanation: **Explanation:** The correct answer is **Gap Junction**. Smooth muscle, particularly the "unitary" or "single-unit" type (found in the gastrointestinal tract, uterus, and small blood vessels), functions as a **functional syncytium**. This coordinated contraction is made possible by gap junctions. **Why Gap Junctions are correct:** Gap junctions are specialized intercellular connections composed of proteins called **connexins**. They form channels that allow the direct passage of ions and small molecules between adjacent cells. In smooth muscle, these junctions provide **low-resistance electrical coupling**, allowing an action potential to spread rapidly from one cell to another, ensuring the muscle layer contracts as a single unit. **Analysis of Incorrect Options:** * **A. Zonula Adherens:** These are "belt-like" anchoring junctions that connect the actin cytoskeleton of adjacent cells. They provide mechanical stability but do not facilitate electrical communication. * **B. Macula Adherens (Desmosomes):** These are "spot-like" junctions that provide strong mechanical attachment by linking intermediate filaments (like desmin). They are prominent in tissues subject to high mechanical stress, like the epidermis and cardiac muscle. * **C. Tight Junctions (Zonula Occludens):** These function as a "seal" to prevent the paracellular movement of water and solutes. They are primarily found in epithelial layers (e.g., blood-brain barrier, renal tubules) rather than muscle tissue. **High-Yield Clinical Pearls for NEET-PG:** * **Cardiac Muscle:** Also contains abundant gap junctions (located in the **intercalated discs**) to facilitate rhythmic contraction. * **Multi-unit Smooth Muscle:** (e.g., Iris of the eye, ciliary muscle) has few to no gap junctions, allowing for fine, independent control of individual muscle fibers. * **Connexin 43:** The most common gap junction protein found in the heart. Mutations in connexins are linked to conditions like Charcot-Marie-Tooth disease and certain types of deafness.

Question 146: Which of the following is true regarding a system which has oscillatory responses?

A. Has a lesser gain
B. Has a greater gain
C. Proportional component
D. Positive feedback system (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Positive Feedback System** In physiology, **positive feedback systems** are characterized by a response that reinforces or amplifies the initial stimulus. This creates a "vicious cycle" or an **oscillatory response** where the output continues to increase or fluctuate until a specific climax or external intervention occurs. Unlike negative feedback, which seeks stability (homeostasis), positive feedback drives the system away from the steady state, leading to instability or rapid physiological changes. **Why other options are incorrect:** * **A & B (Lesser/Greater Gain):** "Gain" refers to the efficiency of a control system (calculated as Correction/Error). While negative feedback systems have a high gain to maintain stability, the term "gain" is primarily used to describe the effectiveness of homeostatic regulation. Oscillatory behavior is a functional characteristic of the feedback loop's direction, not merely the magnitude of its gain. * **C (Proportional Component):** This is a term used in control systems (PID controllers) where the output is proportional to the current error. While it helps in stabilizing negative feedback, it does not inherently define the oscillatory nature of positive feedback loops. **High-Yield Clinical Pearls for NEET-PG:** * **Physiological Positive Feedback:** Most feedback in the body is negative. Key exceptions (Positive) include: * **LH Surge:** Leading to ovulation. * **Oxytocin:** During childbirth (Ferguson reflex). * **Blood Clotting Cascade:** Activation of one factor leads to the activation of many. * **Nerve Action Potential:** Opening of Na+ channels causes more Na+ channels to open (Hodgkin cycle). * **Negative Feedback:** The most common regulatory mechanism (e.g., Thyroxine regulation, BP control via baroreceptors). It is characterized by **stability** rather than oscillation.

Question 147: What is the staircase effect in muscle contraction?

A. After a long period of rest, the initial strength of contraction is less than subsequent contractions. (Correct Answer)
B. After a long period of rest, the initial strength of contraction is greater than subsequent contractions.
C. After a long period of work, the strength of contraction is less than subsequent contractions.
D. After a long period of work, the strength of contraction is greater than subsequent contractions.

Explanation: **Explanation:** The **Staircase Effect**, also known as **Treppe**, is a phenomenon observed in skeletal muscle where the strength of contraction increases progressively during the first few twitches when a muscle is stimulated repeatedly at a constant frequency after a period of quiescence. **1. Why Option A is correct:** The underlying mechanism is primarily related to the **availability of Calcium ($Ca^{2+}$)**. When a muscle begins to contract after a long rest, the sarcoplasmic reticulum (SR) cannot re-sequester all the $Ca^{2+}$ immediately between stimuli. This leads to a gradual buildup of $Ca^{2+}$ in the cytosol. Additionally, the rising temperature within the muscle and the increased rate of enzyme activity (like myosin ATPase) enhance the force of each subsequent contraction until a plateau is reached. **2. Why other options are incorrect:** * **Option B:** This describes the opposite of Treppe. Initial contractions are weaker because the muscle "machinery" is cold and $Ca^{2+}$ levels in the sarcoplasm are at their lowest baseline. * **Options C & D:** These options refer to "work" or fatigue. A decrease in contraction strength after prolonged work is defined as **Muscle Fatigue**, caused by the depletion of glycogen/ATP and the accumulation of lactic acid, which is distinct from the Treppe phenomenon. **NEET-PG High-Yield Pearls:** * **Treppe vs. Summation:** Unlike summation, Treppe occurs even when the muscle relaxes completely between stimuli. * **Bowditch Effect:** This is the cardiac equivalent of the staircase effect, where an increase in heart rate leads to increased force of contraction (Positive Inotropic effect). * **Clinical Correlation:** Treppe is the physiological basis for the "warm-up" period used by athletes to optimize muscle performance.

Question 148: Which hormone acts on nuclear receptors?

A. Calcitonin
B. Thyroxine (Correct Answer)
C. Glucocorticoids
D. Vasopressin

Explanation: **Explanation:** The mechanism of hormone action is determined by the chemical nature of the hormone. Hormones that are lipid-soluble can cross the cell membrane and bind to intracellular receptors, whereas water-soluble hormones bind to cell surface receptors. **Why Thyroxine is correct:** Thyroxine (T4) and Triiodothyronine (T3) are unique. Although derived from the amino acid tyrosine, they are highly lipophilic. They enter the cell via carrier-mediated transport and bind directly to **nuclear receptors** (specifically the Thyroid Hormone Receptor, TR). Once bound, they act as transcription factors, altering gene expression and protein synthesis. **Analysis of Incorrect Options:** * **Calcitonin:** A peptide hormone secreted by the parafollicular C-cells of the thyroid. Being water-soluble, it acts via **G-protein coupled receptors (GPCR)** on the cell membrane, utilizing the cAMP second messenger system. * **Glucocorticoids:** While these are lipid-soluble steroid hormones, they primarily bind to **cytoplasmic receptors**. Upon binding, the hormone-receptor complex translocates into the nucleus. (Note: In many competitive exams, if "Nuclear" is the only option, steroids are grouped there, but Thyroxine is the classic "pure" nuclear receptor example). * **Vasopressin (ADH):** A peptide hormone that acts via membrane-bound GPCRs (V1 receptors use the $IP_3/DAG$ pathway; V2 receptors use the cAMP pathway). **High-Yield NEET-PG Pearls:** 1. **Pure Nuclear Receptors:** Thyroid hormones (T3/T4), Retinoic acid, and Vitamin D. 2. **Cytoplasmic Receptors:** Most Steroid hormones (Glucocorticoids, Mineralocorticoids, Progesterone, Testosterone). Estrogen is an exception and often binds directly in the nucleus. 3. **T3 vs. T4:** T3 is the active form with a much higher affinity for the nuclear receptor than T4. 4. **Speed of Action:** Hormones acting on nuclear receptors have a **lag period** (hours to days) because they require new protein synthesis, unlike membrane-acting hormones which trigger immediate post-translational changes.

Question 149: Transcellular fluid is seen in which location?

A. Body cavities lined by epithelium (Correct Answer)
B. Intracellularly
C. Intercellularly
D. None of the above

Explanation: **Explanation:** **Understanding Transcellular Fluid** Transcellular fluid is a specialized sub-compartment of the **Extracellular Fluid (ECF)**. It is defined as the fluid contained within body cavities that are lined by **epithelial cells**. Unlike interstitial fluid, which is formed by simple filtration, transcellular fluid is often produced by the active secretory activities of these epithelial cells, resulting in a composition that differs significantly from plasma. * **Why Option A is Correct:** Body cavities such as the pleural, peritoneal, and pericardial spaces, as well as the synovial joints and the cerebrospinal fluid (CSF) space, are all lined by specialized epithelium (e.g., mesothelium or ependyma). The fluid within these spaces is the classic definition of transcellular fluid. * **Why Option B is Incorrect:** Intracellular fluid (ICF) refers to the fluid contained *within* the cell membrane. Transcellular fluid is located *outside* the cells, making it a component of the ECF. * **Why Option C is Incorrect:** Intercellular (interstitial) fluid is the fluid that bathes the cells in the tissues. While it is part of the ECF, it is not separated by an epithelial barrier and is distinct from the transcellular compartment. **High-Yield Facts for NEET-PG:** 1. **Volume:** Transcellular fluid accounts for approximately **1–2 liters** (about 1–3% of total body water). 2. **Examples:** CSF, intraocular fluid (aqueous humor), synovial fluid, digestive secretions, and fluid in the serous cavities (pleural, peritoneal). 3. **Clinical Significance:** In pathological states, transcellular fluid can increase significantly, leading to conditions like **ascites, pleural effusion, or joint effusion** (often referred to as "third-spacing").

Question 150: Which of the following is best for measuring total body water?

A. Evan's blue
B. I131
C. Tritium oxide (Correct Answer)
D. P 32

Explanation: **Explanation:** The measurement of body fluid compartments is based on the **Indicator Dilution Principle** ($Volume = \frac{Amount\ of\ substance}{Concentration}$). To measure a specific compartment, the indicator must distribute evenly within that compartment and not cross into others. **Why Tritium Oxide is Correct:** Total Body Water (TBW) includes both intracellular and extracellular fluids. To measure TBW, an indicator must be able to cross all cell membranes and distribute uniformly throughout all fluid compartments. **Tritium oxide ($H^3_2O$)** and **Deuterium oxide ($D_2O$)** are isotopes of water; they behave exactly like physiological water, making them the gold standard for TBW measurement. **Antipyrine** is another non-isotopic substance used for this purpose as it is highly lipid-soluble. **Analysis of Incorrect Options:** * **A. Evan’s Blue (T-1824):** This dye binds strongly to plasma albumin. Since it stays within the vascular system, it is used to measure **Plasma Volume**. * **B. $I^{131}$ (Radio-iodinated Serum Albumin):** Similar to Evan's blue, it remains in the intravascular space and is used to measure **Plasma Volume**. * **D. $P^{32}$ (Radioactive Phosphorus):** This is used to label Red Blood Cells (RBCs) to measure **Total Blood Volume**. **High-Yield Clinical Pearls for NEET-PG:** * **Extracellular Fluid (ECF) Volume:** Measured using Inulin (Gold Standard), Mannitol, or Sucrose. * **Intracellular Fluid (ICF) Volume:** Cannot be measured directly. It is calculated as: $ICF = TBW - ECF$. * **Interstitial Fluid Volume:** Cannot be measured directly. It is calculated as: $ECF - Plasma\ Volume$. * **Rule of Thumb:** TBW is approximately 60% of body weight in males and 50% in females.

Practice by Chapter

Cell Structure and Function

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Membrane Transport Mechanisms

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Bioelectric Phenomena

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Homeostasis and Feedback Mechanisms

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Body Fluid Compartments

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Signal Transduction Mechanisms

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Cell-to-Cell Communication

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Principles of Physiological Measurement

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Osmosis and Osmotic Pressure

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Physiological Adaptation Mechanisms

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