General Physiology — MCQs

General Physiology Indian Medical PG Practice Questions and MCQs

Question 291: What is the approximate ratio of the concentration of sodium ions inside a typical mammalian cell to the concentration of sodium ions outside the cell?

A. 0.5
B. 0.3
C. 0.1 (Correct Answer)
D. 0.01

Explanation: **Explanation:** The concentration gradient of ions across the cell membrane is a fundamental concept in cellular physiology. In a typical mammalian cell, sodium ($Na^+$) is the primary extracellular cation, while potassium ($K^+$) is the primary intracellular cation. **1. Why Option C is Correct:** The typical concentration of sodium ions **inside** the cell (Intracellular Fluid - ICF) is approximately **10–15 mEq/L**, whereas the concentration **outside** the cell (Extracellular Fluid - ECF) is approximately **140–145 mEq/L**. * **Ratio Calculation:** $14 / 140 = 0.1$. This steep gradient is actively maintained by the **$Na^+$-$K^+$ ATPase pump**, which pumps 3 $Na^+$ ions out and 2 $K^+$ ions in per ATP molecule hydrolyzed. **2. Why Other Options are Incorrect:** * **Options A (0.5) and B (0.3):** These values suggest a much higher intracellular sodium concentration than what exists physiologically. Such ratios would imply a failure of the $Na^+$-$K^+$ pump, leading to cellular swelling and death. * **Option D (0.01):** This would imply an intracellular concentration of only 1.4 mEq/L, which is too low. While the gradient is steep, it is not a 100-fold difference (unlike Calcium, where the ratio is closer to 0.0001). **High-Yield Clinical Pearls for NEET-PG:** * **Resting Membrane Potential (RMP):** The $Na^+$ gradient contributes to the RMP, but the membrane is much more permeable to $K^+$ at rest. * **Action Potential:** The rapid upstroke (depolarization) of an action potential is caused by the sudden influx of $Na^+$ ions down this concentration gradient via voltage-gated channels. * **Secondary Active Transport:** This $Na^+$ gradient provides the driving force for the transport of other substances, such as glucose (SGLT) and amino acids. * **Ouabain/Digoxin:** These drugs inhibit the $Na^+$-$K^+$ ATPase, increasing intracellular $Na^+$, which subsequently affects the $Na^+$-$Ca^{2+}$ exchanger.

Question 292: What percentage of body weight does intracellular water constitute?

A. 40% (Correct Answer)
B. 60%
C. 25%
D. 80%

Explanation: **Explanation:** The distribution of body fluids is a fundamental concept in physiology based on the **60-40-20 Rule**. In an average healthy adult male, Total Body Water (TBW) accounts for approximately **60%** of the total body weight. 1. **Intracellular Fluid (ICF):** This is the fluid contained within the cells and constitutes **40%** of the total body weight (or 2/3rd of TBW). This makes **Option A** the correct answer. 2. **Extracellular Fluid (ECF):** This fluid exists outside the cells and constitutes **20%** of the total body weight (or 1/3rd of TBW). ECF is further divided into Interstitial fluid (15%) and Plasma (5%). **Analysis of Incorrect Options:** * **Option B (60%):** This represents the **Total Body Water (TBW)** percentage in an average male, not the intracellular component specifically. * **Option C (25%):** This does not correspond to a standard major fluid compartment division in the 60-40-20 rule. * **Option D (80%):** This is the approximate percentage of water in **newborns**, who have a significantly higher water content compared to adults. **High-Yield Clinical Pearls for NEET-PG:** * **Gender Variation:** Females have a lower TBW percentage (~50%) due to a higher proportion of subcutaneous adipose tissue (fat is hydrophobic). * **Age Variation:** TBW is highest in newborns (75-80%) and lowest in the elderly (approx. 45-50%) due to loss of muscle mass. * **Measurement:** ICF cannot be measured directly. It is calculated as: **ICF = TBW – ECF**. * **Markers:** TBW is measured using **Deuterium oxide (D2O)** or Antipyrene; ECF is measured using **Inulin**, Mannitol, or Sucrose.

Question 293: Decreased protein to lipid ratio is characteristic of which membrane?

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

Explanation: **Explanation:** The protein-to-lipid ratio of a biological membrane is determined by its physiological function. Membranes involved in high metabolic activity or signal transduction (like ion pumping and electron transport) have a high protein content, while membranes acting primarily as electrical insulators have a high lipid content. **Why Myelin Sheath is Correct:** The **myelin sheath** serves as an electrical insulator for axons to facilitate saltatory conduction. To minimize ion leakage and capacitance, it is composed of approximately **80% lipids** and only **20% proteins**. This results in a very **low protein-to-lipid ratio (0.25:1)**, the lowest among biological membranes. **Analysis of Incorrect Options:** * **Inner Mitochondrial Membrane:** This membrane is the site of the Electron Transport Chain (ETC) and ATP synthesis. It is packed with enzymes and carrier proteins, giving it the **highest protein-to-lipid ratio (approx. 3:1 or 75% protein)**. * **Outer Mitochondrial Membrane:** While less protein-dense than the inner membrane, it contains numerous porins and enzymes, maintaining a ratio of roughly 1:1. * **Sarcoplasmic Reticulum:** This membrane is specialized for active calcium transport (via Ca²⁺-ATPase pumps). It has a high protein content (approx. 65%) to support its role in muscle contraction/relaxation. **High-Yield Facts for NEET-PG:** * **Standard Plasma Membrane:** Typically has a 1:1 protein-to-lipid ratio. * **Myelin Composition:** High in **sphingomyelin** and cholesterol. * **Clinical Correlation:** Demyelinating diseases like **Multiple Sclerosis** (CNS) and **Guillain-Barré Syndrome** (PNS) involve the destruction of these lipid-rich layers, leading to slowed nerve conduction.

Question 294: All of the following hormones act through cell surface receptors except?

A. Cortisol (Correct Answer)
B. Insulin
C. Follicle-stimulating hormone (FSH)
D. Thyroid-stimulating hormone (TSH)

Explanation: **Explanation:** The mechanism of action of a hormone is primarily determined by its chemical nature (solubility). Hormones are broadly classified into **water-soluble** (lipophobic) and **lipid-soluble** (lipophilic). **1. Why Cortisol is the Correct Answer:** Cortisol is a **steroid hormone** derived from cholesterol. Being lipid-soluble, it easily diffuses across the phospholipid bilayer of the cell membrane. Therefore, it does not require a cell surface receptor; instead, it binds to **intracellular receptors** (specifically, Type 1 cytoplasmic receptors). The hormone-receptor complex then translocates to the nucleus to alter gene transcription. **2. Why the Other Options are Incorrect:** * **Insulin (Option B):** A peptide hormone that binds to a **Tyrosine Kinase receptor** (an enzyme-linked cell surface receptor). * **FSH and TSH (Options C & D):** Both are glycoprotein hormones. Being large and water-soluble, they cannot cross the cell membrane. They bind to **G-Protein Coupled Receptors (GPCRs)** on the cell surface and utilize the **cAMP** second messenger system. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Intracellular Receptors:** *"**VET** **T**v **C**hannels"* (**V**itamin D, **E**strogen, **T**estosterone, **T**hyroid hormones (T3/T4), **C**ortisol/Aldosterone). * **Exceptions:** While most lipid-soluble hormones use intracellular receptors, **Thyroid hormones** are unique because they bind directly to receptors already located on the **chromatin in the nucleus**. * **Second Messengers:** Remember that **ANP** and **Nitric Oxide** use **cGMP**, while **Catecholamines (α1)** and **Oxytocin** use the **IP3/DAG** pathway.

Question 295: What is the chronaxie minimum in nerves?

A. Mixed nerves
B. Unmyelinated nerve
C. Myelinated nerve (Correct Answer)
D. Sensory nerves

Explanation: ### Explanation **Concept Overview:** Chronaxie is defined as the **minimum time** required to excite a tissue when using a stimulus intensity equal to twice the rheobase (the minimum current required for excitation). It is a measure of **excitability**: the shorter the chronaxie, the more excitable the tissue. **Why Myelinated Nerves are Correct:** Myelinated nerves (like Type A fibers) are designed for rapid signal transmission. Due to the presence of the myelin sheath and the Nodes of Ranvier, these fibers have a high density of voltage-gated sodium channels and low membrane capacitance. This makes them highly excitable, resulting in the **shortest chronaxie** among all nerve types. **Analysis of Incorrect Options:** * **Unmyelinated Nerves:** These fibers (like Type C fibers) lack myelin, leading to slower conduction velocities and lower excitability. Consequently, they have a **longer chronaxie** compared to myelinated fibers. * **Mixed Nerves:** A mixed nerve contains a variety of fibers (myelinated, unmyelinated, sensory, and motor). Its chronaxie would be an average or representative of its constituent parts, but it is not the "minimum" value. * **Sensory Nerves:** While many sensory nerves are myelinated, this category also includes unmyelinated fibers (e.g., slow pain). Therefore, "myelinated nerve" is a more precise physiological descriptor for minimum chronaxie. **High-Yield NEET-PG Pearls:** * **Excitability Relationship:** Chronaxie is inversely proportional to excitability ($Chronaxie \propto 1/Excitability$). * **Order of Chronaxie:** Myelinated Nerve < Unmyelinated Nerve < Skeletal Muscle < Cardiac Muscle < Smooth Muscle. * **Clinical Use:** Chronaxie is used in electrodiagnosis to assess nerve regeneration or muscle denervation (denervated muscles show a significantly increased chronaxie). * **Rheobase:** The minimum strength of current (intensity) required to evoke a response if applied for an infinite duration.

Question 296: What is true about an action potential?

A. It is a decremental phenomenon.
B. It does not obey the all-or-none phenomenon.
C. Potassium ions move from the extracellular fluid to the intracellular fluid.
D. A threshold stimulus is required for its generation. (Correct Answer)

Explanation: ### Explanation **Correct Option: D. A threshold stimulus is required for its generation.** An action potential (AP) is an "all-or-none" electrical event. For an AP to occur, the membrane potential must be depolarized to a specific level known as the **threshold potential** (typically -55 mV in neurons). Once this threshold is reached, voltage-gated sodium channels open rapidly, triggering a self-propagating depolarization. If the stimulus is sub-threshold, no AP is generated. **Analysis of Incorrect Options:** * **A. It is a decremental phenomenon:** This is incorrect. Action potentials are **non-decremental**; they maintain a constant amplitude and shape as they propagate along the axon. Local potentials (like EPSPs or IPSPs), however, are decremental. * **B. It does not obey the all-or-none phenomenon:** This is incorrect. The AP strictly follows the **All-or-None Law**. If the threshold is met, a full-strength AP occurs; if not, nothing happens. Increasing the stimulus strength beyond the threshold does not increase the AP's amplitude. * **C. Potassium ions move from ECF to ICF:** This is incorrect. During the repolarization phase of an AP, voltage-gated **potassium channels open**, causing $K^+$ to move **out of the cell** (from ICF to ECF) down its electrochemical gradient. **High-Yield NEET-PG Pearls:** * **Depolarization phase:** Primarily due to $Na^+$ influx. * **Repolarization phase:** Primarily due to $K^+$ efflux. * **Absolute Refractory Period:** Occurs during the peak of AP when $Na^+$ channels are in an inactivated state; no second AP can be fired regardless of stimulus strength. * **Myelination:** Increases conduction velocity via **Saltatory Conduction** (jumping between Nodes of Ranvier).

Question 297: What is the process by which fusion of particles to a cell membrane occurs?

A. Cell division
B. Endocytosis (Correct Answer)
C. Exocytosis
D. Virus replication

Explanation: **Explanation:** The process by which particles or substances are internalized by a cell through the **fusion and invagination** of the cell membrane is known as **Endocytosis**. During this process, the plasma membrane surrounds the target particle, fuses its edges, and pinches off to form an intracellular vesicle. This is a form of active transport used for large molecules (macromolecules) that cannot pass through channels or transporters. **Analysis of Options:** * **Endocytosis (Correct):** It involves the "fusion" of the membrane to engulf extracellular material. It is categorized into Phagocytosis (cell eating), Pinocytosis (cell drinking), and Receptor-mediated endocytosis (e.g., LDL uptake). * **Exocytosis (Incorrect):** While exocytosis also involves membrane fusion, it is the process of **expelling** materials (like neurotransmitters or hormones) from the cell into the extracellular space. The question specifically refers to the uptake/fusion of external particles to the membrane. * **Cell Division (Incorrect):** This is the process of a parent cell dividing into two daughter cells (Mitosis/Meiosis). While membrane remodeling occurs, it is not a mechanism for particle uptake. * **Virus Replication (Incorrect):** This is a biological cycle occurring *inside* a host cell after entry. While some viruses enter via endocytosis, the replication process itself involves protein synthesis and genomic copying, not the fusion of particles to the membrane. **High-Yield NEET-PG Pearls:** * **Clathrin-coated pits:** Essential for receptor-mediated endocytosis (e.g., uptake of Iron via Transferrin). * **Caveolae:** Small invaginations of the plasma membrane involved in transcytosis and cell signaling. * **SNARE Proteins:** Critical for the fusion of vesicles with the target membrane during exocytosis (Targeted by Botulinum and Tetanus toxins). * **Phagocytosis** is primarily performed by "professional phagocytes" like Neutrophils and Macrophages.

Question 298: What is the role of the liver in vitamin D metabolism?

A. Catalyzes the rate-limiting step
B. 1-hydroxylation
C. 25-hydroxylation (Correct Answer)
D. 24-hydroxylation

Explanation: **Explanation:** The metabolism of Vitamin D is a multi-step process involving the skin, liver, and kidneys. The liver’s primary role is the **25-hydroxylation** of Vitamin D. 1. **Why Option C is Correct:** Once Vitamin D (D3 from skin/diet or D2 from diet) enters the circulation, it is transported to the liver. Here, the enzyme **25-hydroxylase** (a cytochrome P450 enzyme) adds a hydroxyl group to the 25th carbon to form **25-hydroxyvitamin D [25(OH)D]**, also known as **Calcidiol**. This is the major circulating form of Vitamin D and the standard marker used to clinically assess a patient's Vitamin D status. 2. **Why Other Options are Incorrect:** * **Option A:** The rate-limiting step occurs in the **kidneys**, catalyzed by 1-alpha-hydroxylase, which is strictly regulated by Parathyroid Hormone (PTH) and phosphate levels. * **Option B:** 1-hydroxylation occurs in the **proximal convoluted tubules of the kidney** to produce the active form, 1,25-dihydroxyvitamin D (Calcitriol). * **Option D:** 24-hydroxylation is a catabolic pathway occurring in the kidneys that creates 24,25-dihydroxyvitamin D, an inactive metabolite, when Vitamin D levels are sufficient. **NEET-PG High-Yield Pearls:** * **Storage:** Calcidiol (25-OH D) has a long half-life (2-3 weeks), making it the best indicator of body stores. * **Active Form:** Calcitriol (1,25-(OH)₂ D) is the most potent form but has a short half-life (hours). * **Clinical Link:** In chronic liver disease, 25-hydroxylation may be impaired, leading to Vitamin D deficiency and hepatic osteodystrophy.

Question 299: All of the following statements about Myasthenia Gravis are true, except?

A. Anti-AChR antibodies are the most common antibodies in generalized myasthenia gravis.
B. The pathogenic Anti-AChR antibodies are IgG antibodies.
C. Anti-MuSK antibodies are directed against muscarinic ACh receptors. (Correct Answer)
D. The major defect is a decrease in the number of available ACh receptors.

Explanation: ### Explanation **Myasthenia Gravis (MG)** is an autoimmune disorder of the neuromuscular junction (NMJ) characterized by muscle weakness and fatigability. **Why Option C is the Correct Answer (The False Statement):** Anti-MuSK antibodies are directed against **Muscle-Specific Kinase (MuSK)**, not muscarinic receptors. MuSK is a transmembrane protein essential for the clustering and maintenance of **nicotinic acetylcholine receptors (nAChR)** at the motor endplate. Patients with MuSK-positive MG often present with more severe bulbar, facial, and respiratory weakness compared to classic MG. **Analysis of Other Options:** * **Option A:** **True.** Anti-AChR antibodies are found in approximately 85% of patients with generalized MG, making them the most common diagnostic marker. * **Option B:** **True.** The pathogenic antibodies in MG belong to the **IgG class** (specifically IgG1 and IgG3). They cause damage via three mechanisms: complement-mediated destruction of the postsynaptic membrane, increased degradation of receptors (endocytosis), and direct blockade of the ACh binding site. * **Option D:** **True.** The fundamental physiological defect in MG is a **reduction in the number of available postsynaptic nicotinic ACh receptors**, leading to a decreased "safety factor" of neuromuscular transmission. **High-Yield Clinical Pearls for NEET-PG:** * **Lambert-Eaton Myasthenic Syndrome (LEMS):** Contrast MG with LEMS, where antibodies are directed against **presynaptic P/Q-type voltage-gated calcium channels**. * **Thymus Association:** Approximately 75% of MG patients have thymus abnormalities (65% hyperplasia, 10% thymoma). * **Tensilon Test:** Uses **Edrophonium** (short-acting acetylcholinesterase inhibitor) for diagnosis (though now largely replaced by serology and EMG). * **Ice Pack Test:** A simple bedside test where cooling the eyelid improves ptosis by inhibiting acetylcholinesterase activity.

Question 300: Glucose is co-transported with Na+ ions. This is a type of?

A. Secondary active transport (Correct Answer)
B. Primary active transport
C. Facilitated diffusion
D. Simple diffusion

Explanation: **Explanation:** The correct answer is **Secondary Active Transport**. **1. Why it is correct:** Glucose transport via **SGLT (Sodium-Glucose Linked Transporters)** is the classic example of secondary active transport (specifically, symport). In this process, glucose moves against its concentration gradient by "hitching a ride" with Sodium ($Na^+$). The energy is not derived directly from ATP hydrolysis at the transport site; instead, it utilizes the **electrochemical gradient** created by the $Na^+/K^+$ ATPase pump (which maintains low intracellular $Na^+$). Because the transport depends on a gradient established by a primary active process, it is termed "secondary." **2. Why other options are incorrect:** * **Primary Active Transport:** This involves direct hydrolysis of ATP by the carrier protein itself (e.g., $Na^+/K^+$ ATPase, $Ca^{2+}$ ATPase). Glucose transporters do not hydrolyze ATP directly. * **Facilitated Diffusion:** This is a passive process using a carrier protein (e.g., **GLUT** transporters) where molecules move *down* their concentration gradient without energy expenditure. * **Simple Diffusion:** This involves the movement of small, non-polar molecules (like $O_2$ or $CO_2$) directly through the lipid bilayer without the help of a membrane protein. **3. NEET-PG High-Yield Pearls:** * **SGLT-1:** Located in the **Small Intestine** (for glucose absorption) and the late proximal tubule of the kidney. * **SGLT-2:** Located in the **Early Proximal Tubule (S1 segment)** of the kidney; it is responsible for 90% of glucose reabsorption. * **Clinical Correlation:** **SGLT-2 Inhibitors** (e.g., Dapagliflozin) are modern drugs used in Type 2 Diabetes to induce glucosuria and lower blood sugar. * **Oral Rehydration Therapy (ORS):** Works on the principle of $Na^+$-Glucose co-transport; $Na^+$ absorption is enhanced by glucose, which subsequently drags water along osmotically.

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