General Physiology — MCQs

General Physiology Indian Medical PG Practice Questions and MCQs

Question 221: Which substance is most accurate for the measurement of extracellular fluid (ECF) volume?

A. Deuterium oxide
B. Cr-51 labeled RBC
C. Inulin (Correct Answer)
D. Albumin

Explanation: **Explanation:** The measurement of body fluid compartments is based on the **Indicator Dilution Principle** ($Volume = Amount / Concentration$). To measure a specific compartment, the substance used must be able to distribute evenly throughout that compartment but not cross into others. **Why Inulin is the Correct Answer:** **Inulin** is a polysaccharide that is ideal for measuring **Extracellular Fluid (ECF) volume**. It is small enough to pass through capillary pores into the interstitial space but too large and polar to cross cell membranes into the intracellular fluid (ICF). Other substances used for ECF measurement include **Mannitol, Sucrose, and Thiosulfate**. **Analysis of Incorrect Options:** * **A. Deuterium oxide ($D_2O$):** Also known as "Heavy Water," it distributes across all water-containing compartments (ECF + ICF). Therefore, it is used to measure **Total Body Water (TBW)**. * **B. Cr-51 labeled RBC:** These stay strictly within the vascular system. They are used to measure **Total Blood Volume**. * **C. Albumin (Radio-iodinated):** Albumin remains primarily in the intravascular space. It is used to measure **Plasma Volume**. **High-Yield Clinical Pearls for NEET-PG:** * **Intracellular Fluid (ICF):** Cannot be measured directly. It is calculated as $TBW - ECF$. * **Interstitial Fluid:** Cannot be measured directly. It is calculated as $ECF - Plasma\ Volume$. * **Inulin's Dual Role:** Inulin is the "Gold Standard" for measuring both **ECF volume** and **Glomerular Filtration Rate (GFR)** because it is freely filtered but neither reabsorbed nor secreted by the renal tubules.

Question 222: The titin protein present in the muscle fibre binds?

A. The tail ends of actin filaments to Z disc
B. The tail ends of myosin filaments to the Z disc (Correct Answer)
C. The tail end of myosin filaments to M disc
D. The myofibrils to each other, and also to the cell membrane

Explanation: ### Explanation **1. Why Option B is Correct:** Titin (also known as connectin) is the largest known protein in the human body. It acts as a molecular spring that maintains the structural integrity of the sarcomere. One end of the titin molecule is anchored to the **Z-disc**, while the other end extends to the **M-line**. Crucially, the portion of titin between the Z-disc and the beginning of the thick filament is elastic, while the remainder is bound to the **thick (myosin) filament**. Thus, titin serves as a scaffold that anchors the tail ends of myosin filaments to the Z-disc, ensuring they remain centered during contraction and relaxation. **2. Analysis of Incorrect Options:** * **Option A:** Actin filaments are anchored to the Z-disc by **alpha-actinin**, not titin. * **Option C:** While titin does extend to the M-line, its primary functional role in providing "passive elasticity" and structural alignment is defined by its connection between the **myosin filament and the Z-disc**. * **Option D:** The protein responsible for linking myofibrils to each other and to the sarcolemma (cell membrane) is **Desmin** (an intermediate filament) and the **Dystrophin-Glycoprotein Complex**. **3. NEET-PG High-Yield Pearls:** * **Largest Protein:** Titin is the largest single polypeptide chain (approx. 3,800 kDa). * **Passive Tension:** Titin is responsible for the "resting" or passive tension of a muscle when it is stretched. * **Nebulin:** Often confused with titin, Nebulin acts as a "molecular ruler" to regulate the length of **actin** (thin) filaments. * **Clinical Correlation:** Mutations in the titin gene (*TTN*) are a leading cause of **Dilated Cardiomyopathy (DCM)**.

Question 223: The Donnan effect of ion movement across the capillary wall is due to which of the following?

A. Cations
B. Proteins (Correct Answer)
C. Anions
D. Albumin

Explanation: **Explanation:** The **Gibbs-Donnan Effect** describes the behavior of charged particles near a semi-permeable membrane when one of the ions is **non-diffusible**. In the context of the capillary wall, **plasma proteins** (primarily albumin) act as these non-diffusible anions because they are too large to pass through the capillary pores. 1. **Why Proteins are correct:** Plasma proteins carry a net negative charge. Because they cannot cross the capillary wall, they exert an electrostatic pull on diffusible cations (like $Na^+$) to maintain electroneutrality and repel diffusible anions (like $Cl^-$). This results in a higher concentration of cations and a higher osmotic pressure (oncotic pressure) inside the capillary compared to the interstitial fluid. 2. **Why other options are incorrect:** * **Cations (A) and Anions (C):** Most small ions like $Na^+$, $K^+$, and $Cl^-$ are freely diffusible across the capillary wall. The Donnan effect is not *caused* by them, but rather *affects* their distribution due to the presence of the non-diffusible proteins. * **Albumin (D):** While albumin is the most abundant plasma protein contributing to this effect, the term **"Proteins"** is the more comprehensive and standard physiological answer, as globulins and other plasma proteins also contribute to the total non-diffusible anionic charge. **High-Yield Clinical Pearls for NEET-PG:** * **Donnan Factor:** In plasma, the concentration of cations is about **5% higher** than in the interstitial fluid due to this effect. * **Osmotic Implications:** The Donnan effect increases the total osmotic pressure of plasma by approximately **6-7 mmHg** more than what would be exerted by proteins alone. * **Formula:** At equilibrium, the product of diffusible ions on one side equals the product on the other: $[Na^+]_{in} \times [Cl^-]_{in} = [Na^+]_{out} \times [Cl^-]_{out}$.

Question 224: Which statement is true about the lipid bilayer of a cell membrane?

A. It has an asymmetrical arrangement of membrane components. (Correct Answer)
B. It allows for the lateral diffusion of ions.
C. It has a symmetrical arrangement of membrane components.
D. It is not made up of amphipathic lipids.

Explanation: ### Explanation **1. Why Option A is Correct:** The cell membrane is characterized by **membrane asymmetry**. This means the lipid composition of the outer leaflet differs significantly from the inner (cytosolic) leaflet. For example, **Phosphatidylcholine** and **Sphingomyelin** are primarily found in the outer leaflet, while **Phosphatidylserine** and **Phosphatidylethanolamine** are concentrated in the inner leaflet. This asymmetry is crucial for cell signaling, vesicle budding, and preventing unwanted blood clotting. **2. Why the Other Options are Incorrect:** * **Option B:** While the bilayer allows for the lateral diffusion of *lipids and proteins*, it is **impermeable to ions** (like $Na^+$, $K^+$, $Cl^-$) due to its hydrophobic core. Ions require specific transmembrane channels or transporters to cross the membrane. * **Option C:** As explained above, the membrane is asymmetrical. A symmetrical arrangement would impair specific cellular functions, such as apoptosis recognition. * **Option D:** Cell membranes are composed almost entirely of **amphipathic lipids** (phospholipids, cholesterol, and glycolipids). Amphipathic means they possess both a hydrophilic (water-loving) head and a hydrophobic (water-fearing) tail, which is the fundamental property that allows the formation of a bilayer in an aqueous environment. **3. High-Yield NEET-PG Pearls:** * **Flip-Flop Movement:** Lipids rarely move from one leaflet to another spontaneously. This requires specific enzymes: **Flippases** (P-type ATPase, moves lipids in), **Floppases** (ABC transporter, moves lipids out), and **Scramblases** (calcium-dependent, moves lipids bi-directionally). * **Clinical Significance of Phosphatidylserine:** In healthy cells, it is kept in the inner leaflet. Its appearance on the **outer leaflet** is a "eat-me" signal for macrophages, marking the cell for **Apoptosis**. * **Fluidity:** Membrane fluidity increases with high temperatures and a higher proportion of **unsaturated fatty acids** (due to "kinks" in the tails).

Question 225: On changing position from lying down to standing, there is a drop in blood pressure of 10 mm Hg. Immediately, the blood pressure recovers by 8 mm Hg, leaving behind a drop of 2 mm Hg. What is the gain for the baroreceptor system in the control of blood pressure?

A. 2 mm Hg
B. 4 mm Hg (Correct Answer)
C. 8 mm Hg
D. 10 mm Hg

Explanation: ### Explanation The concept of **Feedback Gain** is a measure of the effectiveness of a control system (like the baroreceptor reflex) in maintaining homeostasis. It is calculated using the following formula: $$\text{Gain} = \frac{\text{Correction}}{\text{Residual Error}}$$ **1. Why Option B is Correct:** In this scenario: * **Initial Change (Disturbance):** 10 mm Hg (The drop that would occur without compensation). * **Correction:** 8 mm Hg (The amount the baroreceptor reflex successfully restored). * **Residual Error:** 2 mm Hg (The remaining deviation from the original baseline). Applying the formula: $$\text{Gain} = \frac{8\text{ mm Hg}}{2\text{ mm Hg}} = 4$$ Therefore, the gain of the baroreceptor system in this individual is **4**. **2. Why Other Options are Incorrect:** * **Option A (2 mm Hg):** This represents the **Residual Error**, not the gain. * **Option C (8 mm Hg):** This represents the **Correction** achieved by the system. * **Option D (10 mm Hg):** This is the total **Initial Disturbance** caused by the change in posture. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Definition of Gain:** A higher gain indicates a more efficient regulatory system. * **Negative Feedback:** Most physiological systems (e.g., BP control, body temperature, hormone regulation) operate via negative feedback. The gain for negative feedback systems is technically a negative value (e.g., -4), but in exams, the absolute value is typically used. * **Baroreceptor Reflex:** This is a **short-term** regulator of BP. It is most sensitive at pressures near the normal mean arterial pressure (approx. 95–100 mm Hg). * **Highest Gain System:** The **Renal-Body Fluid System** for long-term blood pressure control has an "infinite gain," meaning it can eventually return the pressure exactly to the starting point (zero residual error).

Question 226: Which of the following are force-generating proteins?

A. Myosin and myoglobin
B. Dynein and kinesin (Correct Answer)
C. Calmodulin and G protein
D. Troponin

Explanation: **Explanation:** The question focuses on **molecular motors**, which are specialized proteins that convert chemical energy (ATP) into mechanical work to generate force and movement within cells. **Why Option B is Correct:** **Dynein and Kinesin** are the primary microtubule-based motor proteins. * **Kinesins** generally move cargo (organelles, vesicles) toward the **plus-end** of microtubules (anterograde transport, e.g., from the cell body to the axon terminal). * **Dyneins** move cargo toward the **minus-end** (retrograde transport) and are also responsible for the beating of cilia and flagella. * *Note:* **Myosin** is also a force-generating protein (actin-based), but it is paired incorrectly in Option A. **Why Other Options are Incorrect:** * **Option A:** While Myosin is a motor protein, **Myoglobin** is an iron-containing protein found in muscle cells that functions solely for oxygen storage, not force generation. * **Option C:** **Calmodulin** is a calcium-binding messenger protein that regulates various enzymatic activities (like MLCK). **G proteins** are molecular switches involved in signal transduction. Neither generates mechanical force. * **Option D:** **Troponin** is a regulatory protein complex (consisting of Troponin C, I, and T) that controls the interaction between actin and myosin. It acts as a "switch" but does not generate force itself. **High-Yield Clinical Pearls for NEET-PG:** * **Kartagener Syndrome:** Caused by a defect in **dynein arms**, leading to immotile cilia, bronchiectasis, and situs inversus. * **Axonal Transport:** Fast axonal transport (400 mm/day) utilizes kinesin and dynein, whereas slow transport (0.5–10 mm/day) carries structural proteins like tubulin. * **Myosin II** is the specific type of myosin responsible for muscle contraction.

Question 227: Antibody specificity is determined by the amino acid sequence within which region?

A. Fc region
B. Constant region
C. Variable region (Correct Answer)
D. Fc receptors

Explanation: **Explanation:** The specificity of an antibody (Immunoglobulin) is determined by its ability to bind to a specific antigen. This binding occurs at the **Variable (V) region**, located at the tips of the "Y" shaped molecule. 1. **Why the Variable Region is Correct:** The variable regions of both the heavy (VH) and light (VL) chains contain **Hypervariable regions** or **Complementarity-Determining Regions (CDRs)**. The unique amino acid sequences in these CDRs create a specific three-dimensional shape that is complementary to a specific epitope on an antigen, much like a lock and key. This sequence diversity is generated through V(D)J recombination during B-cell development. 2. **Why other options are incorrect:** * **Constant (C) region:** This region has a stable amino acid sequence within a particular class of antibody. It determines the **isotype** (IgG, IgM, etc.) and the biological effector function, not antigen specificity. * **Fc region (Fragment crystallizable):** This is the tail region of an antibody formed by the constant domains of the heavy chains. it mediates functions like complement activation and binding to cell surface receptors. * **Fc receptors:** These are proteins found on the surface of certain immune cells (like macrophages and NK cells) that bind to the Fc portion of antibodies; they are not part of the antibody's primary structure. **High-Yield Clinical Pearls for NEET-PG:** * **Fab fragment:** Contains one constant and one variable domain of each of the heavy and light chains; it is the part that binds to the antigen. * **Papain digestion:** Cleaves an antibody into **two Fab** fragments and **one Fc** fragment. * **Pepsin digestion:** Cleaves an antibody into **one F(ab')2** fragment (divalent) and degraded Fc fragments. * **Isotype switching:** Changes the Constant region (e.g., IgM to IgG) but keeps the Variable region (specificity) the same.

Question 228: Which of the following muscles has the maximum density of white muscle fibers?

A. Calf muscle
B. Extraocular muscles (Correct Answer)
C. Back muscles
D. Hip muscles

Explanation: **Explanation:** The classification of muscle fibers into **Red (Type I)** and **White (Type II)** is based on their metabolic profile and contraction speed. White muscle fibers (Type II) are characterized by high myosin ATPase activity, low myoglobin content, and a reliance on anaerobic glycolysis. They are designed for **rapid, short bursts of activity** but fatigue easily. **Why Extraocular Muscles are correct:** The extraocular muscles (EOMs) require the fastest contraction speeds in the human body to perform saccadic eye movements. Consequently, they possess the highest density of **Type II (White) fibers**. These muscles are specialized for precision and extreme velocity rather than sustained weight-bearing or posture, making them the classic example of white muscle predominance. **Analysis of Incorrect Options:** * **Calf Muscle (Gastrocnemius/Soleus):** The Soleus is a classic "Red" muscle (Type I) used for maintaining standing posture. While the Gastrocnemius has mixed fibers, it does not match the white fiber density of the EOMs. * **Back Muscles (Erector Spinae):** These are "Antigravity" muscles. They consist predominantly of Type I (Red) fibers because they must remain contracted for long periods to maintain posture without fatiguing. * **Hip Muscles (Gluteus Maximus):** These are large, powerful muscles used for locomotion and posture. They contain a high proportion of Type I fibers to support sustained activity. **High-Yield Clinical Pearls for NEET-PG:** * **Type I (Red):** "One Slow Red Ox" — Type **I**, **Slow**-twitch, **Red** (high myoglobin), **Ox**idative phosphorylation (high mitochondria). Found in marathon runners. * **Type II (White):** Fast-twitch, glycolytic, low mitochondria. Found in sprinters and extraocular muscles. * **Laryngeal muscles** also have a high density of fast-twitch fibers, second only to extraocular muscles.

Question 229: How is an electrical synapse different from a chemical synapse?

A. There is no cytoplasmic continuity between presynaptic and postsynaptic neurons.
B. The direction of impulse transmission is unidirectional.
C. Connexons are involved in the structure of an electrical synapse. (Correct Answer)
D. Electrical synapses have a longer latent period than chemical synapses.

Explanation: ### Explanation **1. Why the Correct Answer is Right:** Electrical synapses are characterized by the presence of **gap junctions**, which provide direct **cytoplasmic continuity** between the presynaptic and postsynaptic neurons. These gap junctions are composed of specialized protein channels called **Connexons**. Each connexon is made up of six subunits called **connexins**. These channels allow the direct flow of ions and small molecules, resulting in virtually instantaneous signal transmission. **2. Why the Other Options are Wrong:** * **Option A:** In electrical synapses, there **is** cytoplasmic continuity via gap junctions. In contrast, chemical synapses have a distinct gap called the synaptic cleft (20–40 nm) with no physical continuity. * **Option B:** Transmission in electrical synapses is typically **bidirectional**, allowing signals to flow in both directions. Chemical synapses are strictly **unidirectional** due to the specific arrangement of neurotransmitter release and receptor binding. * **Option C:** (Correct) As explained above, connexons are the structural hallmarks of electrical synapses. * **Option D:** Electrical synapses have a **shorter (virtually zero) latent period** because they do not involve the time-consuming steps of neurotransmitter release, diffusion, and receptor activation seen in chemical synapses (which have a "synaptic delay" of ~0.5 ms). **3. NEET-PG High-Yield Pearls:** * **Fastest Transmission:** Electrical synapses are the fastest in the body; they are found where rapid, synchronized activity is required (e.g., cardiac muscle, smooth muscle, and certain brainstem nuclei). * **Synaptic Delay:** Chemical synapses are the most common type in the human CNS and are the primary site for **synaptic plasticity** and drug action. * **Connexin Mutations:** Mutations in connexin genes are linked to clinical conditions like **Charcot-Marie-Tooth disease** (Cx32) and certain types of **congenital deafness** (Cx26).

Question 230: Which of the following is NOT a component of the thin filament?

A. Actin
B. Troponin
C. Myosin (Correct Answer)
D. Tropomyosin

Explanation: ### Explanation The sarcomere, the functional unit of skeletal muscle, is composed of thick and thin filaments. Understanding their molecular composition is fundamental to the sliding filament theory of muscle contraction. **Why Myosin is the Correct Answer:** **Myosin** is the primary constituent of the **thick filament**, not the thin filament. A myosin molecule consists of two heavy chains (forming the tail and heads) and four light chains. The myosin heads contain ATPase activity and binding sites for actin, which are essential for cross-bridge formation. **Analysis of Incorrect Options (Components of the Thin Filament):** * **Actin (Option A):** This is the backbone of the thin filament. It exists as globular (G-actin) monomers that polymerize to form filamentous (F-actin) double-helical strands. * **Troponin (Option B):** A complex of three regulatory proteins: **Troponin T** (binds to tropomyosin), **Troponin I** (inhibits actin-myosin binding), and **Troponin C** (binds calcium ions). * **Tropomyosin (Option D):** A regulatory protein that wraps around the actin helix. In a resting state, it physically covers the active sites on actin, preventing interaction with myosin heads. **High-Yield NEET-PG Pearls:** * **Regulatory Proteins:** Troponin and Tropomyosin are termed "regulatory proteins" because they control the "on/off" switch for contraction. * **Contractile Proteins:** Actin and Myosin are the "contractile proteins." * **Structural Proteins:** **Titin** (the largest known protein) anchors myosin to the Z-discs, while **Nebulin** acts as a molecular ruler for actin length. * **Clinical Correlation:** **Troponin I and T** are highly specific cardiac biomarkers used in the diagnosis of Myocardial Infarction (MI).

Practice by Chapter

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Signal Transduction Mechanisms

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