General Physiology — MCQs

General Physiology Indian Medical PG Practice Questions and MCQs

Question 241: Which of the following membranes has the highest protein content per gram of tissue?

A. Inner mitochondrial membrane (Correct Answer)
B. Outer mitochondrial membrane
C. Plasma membrane
D. Myelin sheath

Explanation: The protein-to-lipid ratio of biological membranes varies significantly based on their physiological function. Proteins are responsible for active processes like transport, enzymatic reactions, and energy production, while lipids provide structural insulation. ### **Explanation of the Correct Answer** **A. Inner Mitochondrial Membrane (IMM):** This membrane has the highest protein content in the body (approximately **75-80% protein** and 20-25% lipid). This high density is due to the presence of the **Electron Transport Chain (ETC)** complexes, ATP synthase, and various transport proteins required for cellular respiration. The IMM is also unique because it contains **cardiolipin**, which makes it impermeable to most ions, necessitating a high number of specific transporter proteins. ### **Analysis of Incorrect Options** * **B. Outer Mitochondrial Membrane:** While it contains proteins like porins, its protein-to-lipid ratio is roughly **50:50**, similar to many other organelle membranes. * **C. Plasma Membrane:** Most plasma membranes (e.g., in RBCs) have a balanced ratio of approximately **50% protein and 50% lipid**. * **D. Myelin Sheath:** This membrane has the **lowest protein content** (approx. **20% protein** and 80% lipid). Its primary function is electrical insulation to facilitate saltatory conduction, which requires a high lipid (sphingomyelin) content rather than metabolic machinery. ### **High-Yield Clinical Pearls for NEET-PG** * **Highest Protein Content:** Inner Mitochondrial Membrane (~80%). * **Highest Lipid Content:** Myelin Sheath (~80%). * **Cardiolipin:** A phospholipid found almost exclusively in the IMM; its deficiency is seen in **Barth Syndrome**. * **Marker Enzyme for Mitochondria:** Succinate dehydrogenase (Inner membrane) and Monoamine oxidase (Outer membrane).

Question 242: In an adult man weighing 70 kg, what is the approximate extracellular fluid volume?

A. 42 L
B. 25 L
C. 14 L (Correct Answer)
D. 12 L

Explanation: **Explanation:** The distribution of body fluids is a fundamental concept in physiology based on the **"60-40-20 Rule."** In an average adult male, Total Body Water (TBW) constitutes approximately **60%** of the total body weight. 1. **Total Body Water (TBW):** 60% of 70 kg = **42 L**. 2. **Intracellular Fluid (ICF):** 2/3rd of TBW (40% of body weight) = **28 L**. 3. **Extracellular Fluid (ECF):** 1/3rd of TBW (20% of body weight) = **14 L**. Therefore, for a 70 kg man, the ECF volume is calculated as: $70 \times 0.20 = 14\text{ L}$. **Analysis of Options:** * **Option A (42 L):** This represents the **Total Body Water (TBW)**, not the ECF. * **Option B (25 L):** This is close to the **Intracellular Fluid (ICF)** volume (approx. 28 L). * **Option D (12 L):** While the Interstitial Fluid (a sub-component of ECF) is roughly 11–12 L (15% of body weight), it does not account for the total ECF. **Clinical Pearls & High-Yield Facts:** * **ECF Components:** ECF is further divided into **Interstitial Fluid (3/4th of ECF)** and **Plasma (1/4th of ECF)**. For a 70 kg man, Plasma volume is ~3.5 L. * **Measurement (Indicator Dilution Method):** * **TBW:** Measured using Tritiated water ($^3\text{H}_2\text{O}$) or Deuterium oxide ($^2\text{H}_2\text{O}$). * **ECF:** Measured using **Inulin** (Gold Standard), Mannitol, or Sucrose. * **Plasma Volume:** Measured using **Evans Blue dye** (T-1824) or Radio-iodinated Albumin. * **Gender/Age Variation:** TBW is lower in females (approx. 50%) due to higher subcutaneous fat and highest in newborns (approx. 75%).

Question 243: High anion gap acidosis is seen in all except?

A. Diabetic ketoacidosis
B. Lactic acidosis
C. Renal tubular acidosis (Correct Answer)
D. Methanol poisoning

Explanation: **Explanation:** Metabolic acidosis is classified based on the **Anion Gap (AG)**, calculated as: $AG = [Na^+] - ([Cl^-] + [HCO_3^-])$. The normal range is 8–12 mEq/L. **Why Renal Tubular Acidosis (RTA) is the correct answer:** RTA is a classic cause of **Normal Anion Gap Metabolic Acidosis (NAGMA)**, also known as hyperchloremic metabolic acidosis. In RTA, there is either a failure to reabsorb bicarbonate (Type 2) or a failure to excrete hydrogen ions (Type 1/4). To maintain electroneutrality as bicarbonate is lost, the kidneys retain **Chloride ($Cl^-$)**. Since the increase in chloride offsets the decrease in bicarbonate, the anion gap remains within the normal range. **Analysis of Incorrect Options (Causes of HAGMA):** High Anion Gap Metabolic Acidosis (HAGMA) occurs when "unmeasured anions" (fixed acids) accumulate in the blood. * **Diabetic Ketoacidosis (DKA):** Accumulation of acetoacetate and beta-hydroxybutyrate. * **Lactic Acidosis:** Accumulation of lactate due to tissue hypoxia or sepsis. * **Methanol Poisoning:** Metabolism of methanol into formic acid. **NEET-PG High-Yield Pearls:** * **Mnemonic for HAGMA:** **MUDPILES** (Methanol, Uremia, DKA, Paraldehyde, INH/Iron, Lactic acidosis, Ethylene glycol, Salicylates). * **Mnemonic for NAGMA:** **HARDUP** (Hyperalimentation, Acetazolamide, **RTA**, Diarrhea, Uretero-sigmoidostomy, Pancreatic fistula). * **Key Distinction:** Diarrhea and RTA are the two most common causes of NAGMA tested in exams. Diarrhea involves GI loss of $HCO_3^-$, while RTA involves renal loss/retention issues.

Question 244: What happens when sodium enters cells?

A. There is a spike in action potential. (Correct Answer)
B. There is a plateau in action potential.
C. There is repolarization.
D. There is hyperpolarization.

Explanation: ### Explanation The correct answer is **A. There is a spike in action potential.** **1. Why Option A is correct:** The generation of an action potential is primarily driven by the movement of ions across the cell membrane. When a stimulus reaches the threshold potential, **voltage-gated sodium (Na⁺) channels** open rapidly. Since Na⁺ concentration is much higher extracellularly, it rushes into the cell following its electrochemical gradient. This influx of positive charge causes **depolarization**, making the membrane potential more positive. This rapid upstroke is known as the **"spike"** of the action potential. **2. Why other options are incorrect:** * **B. Plateau in action potential:** This is characteristic of cardiac ventricular muscle cells (Phase 2), caused by the balanced influx of **Calcium (Ca²⁺)** through L-type channels and the efflux of Potassium (K⁺). * **C. Repolarization:** This occurs when the membrane potential returns to its resting state, primarily due to the **efflux of Potassium (K⁺)** out of the cell and the closure of Na⁺ channels. * **D. Hyperpolarization:** This happens when the membrane potential becomes more negative than the resting membrane potential, usually due to an **excessive efflux of K⁺** or an **influx of Chloride (Cl⁻)**. **3. NEET-PG High-Yield Pearls:** * **Tetrodotoxin (from Pufferfish) and Saxitoxin:** Block voltage-gated Na⁺ channels, preventing the spike/depolarization. * **Local Anesthetics (e.g., Lidocaine):** Work by blocking voltage-gated Na⁺ channels from the inside, inhibiting signal conduction. * **Overshoot:** The portion of the action potential where the membrane potential is positive (above 0 mV) is called the overshoot. * **Na⁺-K⁺ ATPase:** This pump does not create the action potential but maintains the ionic gradients necessary for it to occur (3 Na⁺ out, 2 K⁺ in).

Question 245: The sites where myosin heads bind to actin in skeletal muscles are covered by which of the following?

A. Tropomyosin (Correct Answer)
B. Troponin
C. Calcium ions
D. Calmodulin

Explanation: ### Explanation **Correct Answer: A. Tropomyosin** In skeletal muscle, the interaction between actin and myosin is the fundamental step for contraction. In a resting (relaxed) state, the **active sites on the F-actin strand**—where myosin heads must attach to form cross-bridges—are physically blocked. This masking is performed by **Tropomyosin**, a long, rod-like protein that wraps around the actin filament. As long as tropomyosin remains in this position, the myosin heads cannot bind to actin, preventing contraction. **Why other options are incorrect:** * **B. Troponin:** This is a complex of three subunits (I, T, and C). While it is attached to tropomyosin, its role is to act as a "switch." When calcium binds to Troponin C, it undergoes a conformational change that pulls the tropomyosin away from the binding sites. It does not cover the sites itself. * **C. Calcium ions:** Calcium is the trigger for contraction. It binds to Troponin C to *uncover* the binding sites; it does not cover them. * **D. Calmodulin:** This is the calcium-binding protein used in **smooth muscle** contraction (where troponin is absent). It does not play a structural role in masking actin sites in skeletal muscle. **High-Yield NEET-PG Pearls:** * **The Troponin Complex:** * **Troponin T:** Binds to **T**ropomyosin. * **Troponin I:** **I**nhibits the actin-myosin interaction. * **Troponin C:** Binds **C**alcium ions (up to 4 ions per molecule). * **The "Walk-along" Theory:** Contraction occurs when tropomyosin moves into the "groove" of the actin helix, exposing the sites. * **Clinical Correlation:** Cardiac Troponins (I and T) are highly specific biomarkers for myocardial infarction because they are released into the blood when cardiac myocytes are damaged.

Question 246: Ceruloplasmin binds which of the following?

A. Iron
B. Copper (Correct Answer)
C. Zinc
D. Manganese

Explanation: **Explanation:** **Ceruloplasmin** is an alpha-2 globulin synthesized in the liver. It serves as the primary carrier protein for **Copper** in the plasma, binding approximately 95% of circulating copper. Each molecule of ceruloplasmin can bind six to eight copper atoms. Beyond transport, ceruloplasmin functions as a **ferroxidase enzyme**. It oxidizes ferrous iron ($Fe^{2+}$) to ferric iron ($Fe^{3+}$), which is a crucial step for iron to bind to transferrin for transport in the blood. Therefore, while it interacts with iron metabolism, its primary structural binding partner is copper. **Analysis of Incorrect Options:** * **Option A (Iron):** Iron is primarily transported by **Transferrin** and stored by **Ferritin**. Ceruloplasmin aids iron metabolism via its ferroxidase activity but does not "bind" it as a carrier. * **Option C (Zinc):** Zinc is mainly transported by **Albumin** (approx. 60%) and alpha-2 macroglobulin. * **Option D (Manganese):** Manganese is transported in the blood bound to **Transmanganin** (a beta-1 globulin) and albumin. **Clinical Pearls for NEET-PG:** * **Wilson’s Disease:** Characterized by a deficiency of ceruloplasmin due to a defect in the *ATP7B* gene. This leads to copper deposition in the liver (cirrhosis), brain (basal ganglia), and eyes (**Kayser-Fleischer rings**). * **Menkes Disease:** An X-linked recessive disorder (defect in *ATP7A*) resulting in impaired copper absorption and low ceruloplasmin levels, leading to "kinky hair" and neurological issues. * **Acute Phase Reactant:** Ceruloplasmin levels increase during inflammation, infection, or trauma.

Question 247: Which of the following cells are equivalent to macrophages?

A. Keratinocytes
B. Histiocytes (Correct Answer)
C. Mast cells
D. Microphages

Explanation: **Explanation:** The correct answer is **Histiocytes**. This question tests your knowledge of the **Mononuclear Phagocyte System (MPS)**, a collection of phagocytic cells derived from bone marrow monocytes that reside in various tissues to provide immune surveillance. **1. Why Histiocytes are correct:** Macrophages are mature monocytes that have migrated into tissues. When these cells reside specifically within **connective tissue**, they are termed **Histiocytes**. They function as professional phagocytes, engulfing cellular debris and pathogens, and act as antigen-presenting cells (APCs). **2. Analysis of Incorrect Options:** * **Keratinocytes (A):** These are the primary structural cells of the epidermis. While they can produce cytokines, their main role is forming the skin barrier, not phagocytosis. * **Mast cells (C):** These are myeloid cells found in connective tissue, but they are mediators of **Type I Hypersensitivity** reactions. They contain granules rich in histamine and heparin; they are not considered part of the macrophage lineage. * **Microphages (D):** This is an older term used to describe **Neutrophils**. While neutrophils are phagocytic, they are short-lived "first responders" and are distinct from the long-lived mononuclear macrophage system. **3. High-Yield NEET-PG Clinical Pearls:** To excel in General Physiology and Pathology, memorize the tissue-specific names of macrophages: * **Liver:** Kupffer cells * **CNS:** Microglia * **Lungs:** Alveolar macrophages (Dust cells) * **Bone:** Osteoclasts * **Kidney:** Mesangial cells * **Skin:** Langerhans cells (specifically APCs) * **Placenta:** Hofbauer cells **Key Concept:** All these cells originate from **CD14+ monocytes** in the blood.

Question 248: Following a tetanizing stimulus to a muscle, there is a constant contraction. This is due to:

A. Recruitment phenomenon
B. At high stimulus frequency all muscle fibers are contracting (Correct Answer)
C. Contraction of different muscle fibers at different times
D. All of the above

Explanation: ### Explanation **Core Concept: Tetanization** Tetanization occurs when a muscle is stimulated at such a high frequency that the individual muscle twitches fuse into a single, sustained contraction. This happens because the high frequency of action potentials prevents the sarcoplasmic reticulum from resequestering calcium ions ($Ca^{2+}$) between stimuli. Consequently, the cytosolic $Ca^{2+}$ concentration remains high, keeping the actin-myosin binding sites exposed. **Why Option B is Correct:** For a muscle to achieve a state of maximal, constant contraction (tetanus), the stimulus frequency must be high enough to overcome the relaxation phase of all motor units. At this "critical frequency," **all available muscle fibers are recruited and contracting simultaneously**. The summation of these individual fiber contractions results in a smooth, forceful, and sustained tension. **Analysis of Incorrect Options:** * **Option A (Recruitment phenomenon):** Recruitment (Multiple Motor Unit Summation) refers to increasing the *number* of active motor units to increase force. While recruitment occurs during the buildup to tetanus, the "constant" nature of tetanus specifically depends on the *frequency* of stimulation (Temporal Summation) rather than just the number of fibers. * **Option B (Contraction of different fibers at different times):** This describes **Asynchronous Recruitment**, which is how the body maintains posture and prevents fatigue during submaximal contractions. In tetanus, fibers contract synchronously to maintain maximal tension. **High-Yield NEET-PG Pearls:** * **Treppe (Staircase Phenomenon):** Increase in tension over the first few twitches due to increased $Ca^{2+}$ availability and warming of the muscle. * **Critical Fusion Frequency:** The minimum frequency of stimulation required to produce a smooth tetanus. * **Tetanus vs. Tetany:** Tetanus is a physiological response to high-frequency stimuli; **Tetany** is a clinical condition (often due to hypocalcemia) characterized by involuntary muscle spasms.

Question 249: The equilibrium potential of the resting membrane for a given electrolyte is described by which equation?

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

Explanation: **Explanation:** The **Nernst Equation** is the fundamental formula used to calculate the **equilibrium potential** (also called the Nernst potential) for a **single ion** across a semi-permeable membrane. It represents the electrical potential that exactly balances the chemical concentration gradient of that specific ion, resulting in no net movement of the ion into or out of the cell. **Analysis of Options:** * **A. Nernst Equation (Correct):** It calculates the potential for one specific ion (e.g., $E_{Na^+}$, $E_{K^+}$). The formula is $E = \frac{61}{z} \times \log \frac{[Ion]_{out}}{[Ion]_{in}}$ at body temperature. * **B. Goldman-Hodgkin-Katz (GHK) Equation:** Unlike the Nernst equation, this calculates the **Resting Membrane Potential (RMP)** by considering the concentrations and **relative permeabilities** of *all* major ions (Na⁺, K⁺, and Cl⁻) simultaneously. * **C. Faraday’s Law:** This relates to electromagnetism and electrochemistry (the amount of substance produced at an electrode during electrolysis), which is not used to calculate membrane potentials. * **D. Donnan-Gibbs Equation:** This describes the behavior of charged particles near a semi-permeable membrane when non-diffusible ions (like intracellular proteins) are present, leading to an uneven distribution of diffusible ions. **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 because the membrane is most permeable to it at rest. * **Na⁺ Equilibrium Potential:** Approximately **+61 mV**. * **Hypokalemia:** Moves the RMP further from the threshold (hyperpolarization), making cells less excitable (leading to muscle weakness). * **Hyperkalemia:** Moves the RMP closer to the threshold (depolarization), initially increasing excitability but eventually causing inactivation of Na⁺ channels (leading to cardiac arrhythmias).

Question 250: All of the following are true about cell membrane lipids except?

A. Amphipathic in nature
B. Exhibit lateral diffusion along with proteins
C. Exhibit symmetrical arrangement (Correct Answer)
D. Form bilayer

Explanation: ### Explanation The cell membrane is a dynamic, organized structure primarily composed of a phospholipid bilayer. The correct answer is **C (Exhibit symmetrical arrangement)** because cell membrane lipids are **asymmetrically distributed** between the inner and outer leaflets. #### Why Option C is Correct (The Concept of Asymmetry) The lipid bilayer is inherently **asymmetrical**. Different phospholipids are localized to specific sides: * **Outer Leaflet:** Predominantly contains Phosphatidylcholine and Sphingomyelin. * **Inner (Cytosolic) Leaflet:** Predominantly contains **Phosphatidylserine** and Phosphatidylethanolamine. * **Significance:** This asymmetry is vital for cell signaling. For example, the flipping of Phosphatidylserine to the outer leaflet is a classic clinical marker for **apoptosis** (programmed cell death). #### Why Other Options are Incorrect * **A. Amphipathic in nature:** Lipids have both a hydrophilic (water-loving) polar head and a hydrophobic (water-fearing) non-polar tail. This property is essential for forming biological barriers. * **B. Exhibit lateral diffusion:** According to the **Fluid Mosaic Model** (Singer and Nicolson), lipids and proteins move freely within the plane of the membrane (lateral movement). However, "flip-flop" movement (transverse diffusion) between layers is rare and requires enzymes like flippases. * **D. Form bilayer:** In aqueous environments, amphipathic lipids spontaneously organize into a bilayer to shield their hydrophobic tails from water. #### High-Yield NEET-PG Pearls * **Fluidity:** Increased by high temperature and unsaturated fatty acids (kinks in tails); decreased by cholesterol (at body temperature). * **Carbohydrates:** Glycolipids and glycoproteins are found **exclusively on the outer surface**, forming the glycocalyx. * **Flippases vs. Floppases:** Flippases move lipids inward (P-type ATPase), Floppases move them outward (ABC transporter), and Scramblases move them bidirectionally (calcium-dependent).

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