General Physiology — MCQs

General Physiology Indian Medical PG Practice Questions and MCQs

Question 251: True about the relative refractory period of an action potential?

A. An action potential cannot be generated at all.
B. It is due to the closure of inactive gates of sodium channels.
C. It is due to the opening of potassium channels. (Correct Answer)
D. All of the above.

Explanation: ### Explanation The **Relative Refractory Period (RRP)** is the interval immediately following the Absolute Refractory Period (ARP) during which a second action potential can be evoked, but only by a stimulus that is stronger than the normal threshold. **1. Why Option C is Correct:** The RRP coincides with the period of **delayed outward potassium ($K^+$) conductance**. During repolarization, voltage-gated $K^+$ channels are wide open, causing $K^+$ to leave the cell. This creates a "hyperpolarizing" force that opposes depolarization. To trigger a new action potential, a stimulus must be strong enough to overcome this outward $K^+$ flow and open enough "recovered" $Na^+$ channels to reach the threshold. **2. Why Other Options are Incorrect:** * **Option A:** This describes the **Absolute Refractory Period (ARP)**. In ARP, no stimulus, regardless of strength, can excite the nerve because $Na^+$ channels are either already open or in an inactivated state. * **Option B:** The closure of inactivation gates ($h$-gates) of $Na^+$ channels is the hallmark of the **Absolute Refractory Period**. The transition from ARP to RRP occurs when these inactivation gates begin to reopen (and activation gates close), making the channels "excitable" again. **3. High-Yield Facts for NEET-PG:** * **ARP vs. RRP Timing:** ARP corresponds to the period from the start of the upstroke until roughly one-third of repolarization is complete. RRP lasts from the end of ARP until the membrane returns to its resting potential. * **Accommodation:** If a nerve is subjected to a slowly rising current, the threshold for firing increases. This is due to the slow inactivation of $Na^+$ channels and the opening of $K^+$ channels, similar to the ionic environment of the RRP. * **Clinical Significance:** The refractory period ensures the **unidirectional propagation** of action potentials and limits the maximum frequency of nerve impulses.

Question 252: Which of the following is NOT a characteristic of facilitated diffusion?

A. Vmax
B. Competitive inhibition
C. Requires energy (Correct Answer)
D. Specificity

Explanation: **Explanation:** Facilitated diffusion is a form of **carrier-mediated passive transport**. The correct answer is **C (Requires energy)** because facilitated diffusion occurs along a concentration gradient (from high to low concentration) and therefore does not require metabolic energy (ATP). **Why the other options are characteristics of Facilitated Diffusion:** * **Vmax (Saturation):** Unlike simple diffusion, facilitated diffusion relies on carrier proteins. Once all available carriers are occupied, the rate of transport reaches a plateau (Vmax). This is known as saturation kinetics. * **Competitive Inhibition:** Since transport depends on specific binding sites on carrier proteins, molecules with similar structures can compete for the same site, reducing the transport rate of the primary substance. * **Specificity:** Carrier proteins are highly selective. For example, the GLUT (Glucose Transporter) family specifically transports glucose and closely related hexoses, but not other molecules. **High-Yield Facts for NEET-PG:** 1. **Key Example:** The transport of glucose into skeletal muscle and adipose tissue via **GLUT-4** (which is insulin-dependent) is the classic example of facilitated diffusion. 2. **Distinction:** Both facilitated diffusion and active transport show saturation, specificity, and competition. The **only** difference is that active transport moves substances *against* a gradient and requires ATP. 3. **Simple vs. Facilitated:** Simple diffusion is the only transport mechanism that does not show Vmax (it is limited only by the concentration gradient and surface area).

Question 253: What is the characteristic feature of apoptosis?

A. Cell membrane intact (Correct Answer)
B. Cytoplasmic eosinophilia
C. Nuclear moulding
D. Cell swelling

Explanation: **Explanation:** **Apoptosis**, often referred to as "programmed cell death," is a highly regulated process of cell suicide. The hallmark of apoptosis is that it occurs without an inflammatory response because the **cell membrane remains intact** until the very end. 1. **Why Option A is Correct:** In apoptosis, the cell shrinks (pyknosis) and the chromatin condenses, but the plasma membrane does not rupture. Instead, it undergoes "blebbing" to form **apoptotic bodies**. These membrane-bound vesicles contain intact organelles and nuclear fragments, which are quickly recognized and phagocytosed by macrophages. Because the intracellular contents (like lysosomal enzymes) are never leaked into the extracellular space, there is no secondary inflammation. 2. **Why Incorrect Options are Wrong:** * **B. Cytoplasmic eosinophilia:** While seen in both, it is more characteristic of **Necrosis**. In necrosis, denatured proteins bind to eosin, and the loss of cytoplasmic RNA (which is basophilic) makes the cell appear more pink/red. * **C. Nuclear moulding:** This is a cytological feature typically associated with **Small Cell Carcinoma of the lung**, where nuclei of adjacent cells press against each other, distorting their shapes. It is not a feature of apoptosis. * **D. Cell swelling:** This is the hallmark of **Necrosis** (oncosis) and reversible cell injury. In contrast, apoptosis is characterized by **cell shrinkage**. **High-Yield NEET-PG Pearls:** * **Gold Standard for Detection:** The **TUNEL assay** (Terminal deoxynucleotidyl transferase dUTP nick end labeling) detects DNA fragmentation. * **Morphological Hallmark:** Chromatin condensation (most characteristic). * **Biochemical Hallmark:** Caspase activation and DNA laddering (180-200 base pair fragments). * **Key Marker:** **Annexin V** binds to Phosphatidylserine, which flips from the inner to the outer leaflet of the membrane during apoptosis.

Question 254: Which of the following is an example of primary active transport?

A. Na+-Glucose contransport
B. Movement of water across a membrane
C. Na+-K+ ATPase (Correct Answer)
D. Na+-H+ antiport

Explanation: **Explanation:** **1. Why Na+-K+ ATPase is correct:** Primary active transport is a process where molecules are moved against their electrochemical gradient using energy derived **directly** from the hydrolysis of ATP. The **Na+-K+ ATPase (Sodium-Potassium Pump)** is the classic example. It uses an integral membrane protein with ATPase activity to pump **3 Na+ ions out** of the cell and **2 K+ ions into** the cell per ATP molecule hydrolyzed. This maintains the resting membrane potential and cell volume. **2. Why the other options are incorrect:** * **A & D (Na+-Glucose cotransport and Na+-H+ antiport):** These are examples of **Secondary Active Transport**. They do not use ATP directly. Instead, they utilize the energy stored in the sodium concentration gradient (created by the Na+-K+ ATPase) to move other solutes. Cotransport (symport) moves solutes in the same direction as Na+, while antiport (counter-transport) moves them in the opposite direction. * **B (Movement of water):** Water moves across membranes via **Osmosis**, which is a form of **Passive Transport** (facilitated diffusion through aquaporins). It requires no energy and follows the osmotic pressure gradient. **High-Yield NEET-PG Clinical Pearls:** * **Digitalis/Digoxin:** Inhibits the Na+-K+ ATPase, leading to increased intracellular Na+, which subsequently slows the Na+-Ca2+ exchanger, increasing intracellular Ca2+ and myocardial contractility. * **Stoichiometry:** Remember the "3-2-1" rule: **3** Na+ out, **2** K+ in, **1** ATP used. * **Other Primary Active Transporters:** Ca2+ ATPase (SERCA pump) and H+-K+ ATPase (Proton pump in gastric parietal cells).

Question 255: Which ion causes repolarization?

A. Magnesium
B. Calcium
C. Potassium (Correct Answer)
D. Sodium

Explanation: **Explanation:** The resting membrane potential of a cell is primarily maintained by high intracellular potassium ($K^+$) and high extracellular sodium ($Na^+$). Repolarization is the process by which the cell membrane potential returns to its negative resting state following depolarization. **Why Potassium is Correct:** During the action potential, once the peak is reached, voltage-gated $Na^+$ channels close (inactivation) and **voltage-gated $K^+$ channels open**. This leads to an **efflux (outward movement)** of $K^+$ ions down their electrochemical gradient. As positive charges leave the cell, the interior becomes more negative, effectively restoring the resting membrane potential. This process is known as repolarization. **Why Other Options are Incorrect:** * **Sodium (D):** Sodium influx (entry into the cell) is responsible for **depolarization**, making the membrane potential more positive. * **Calcium (B):** Calcium influx is primarily involved in the **plateau phase** of the cardiac action potential and triggers neurotransmitter release or muscle contraction. * **Magnesium (A):** Magnesium acts as a cofactor for the $Na^+/K^+$ ATPase pump and serves as a natural calcium channel blocker, but it does not directly mediate the repolarization phase. **High-Yield Clinical Pearls for NEET-PG:** * **Hyperkalemia:** Leads to faster repolarization, manifesting as **tall tented T-waves** on an ECG. * **Hypokalemia:** Delays repolarization, leading to flattened T-waves and the appearance of **U-waves**. * The **$Na^+/K^+$ ATPase pump** is electrogenic (3 $Na^+$ out, 2 $K^+$ in) and is essential for maintaining the ionic gradients necessary for these potentials to occur.

Question 256: Which of the following steps is passive and does not depend on ATP distribution?

A. Cotransport of Na+ with molecules
B. Entry of H2O via osmosis (Correct Answer)
C. Exit of Na+ and entry of K+ into the cell
D. Uptake of a molecule by endocytosis

Explanation: **Explanation:** The core concept tested here is the classification of membrane transport mechanisms based on energy requirements. **Why Option B is Correct:** **Osmosis** is the net movement of water molecules across a semi-permeable membrane from a region of lower solute concentration to a region of higher solute concentration. It is a form of **passive transport**, meaning it occurs down a concentration gradient and does not require metabolic energy (ATP). Water moves through the lipid bilayer or specialized channels called **aquaporins** driven solely by osmotic pressure. **Why Other Options are Incorrect:** * **Option A (Cotransport):** This is **Secondary Active Transport**. While it doesn't use ATP directly at the site of transport, it relies on the Na+ gradient created by the Na+-K+ ATPase pump. Therefore, it is indirectly dependent on ATP. * **Option C (Na+/K+ Pump):** This is the classic example of **Primary Active Transport**. The Na+-K+ ATPase pump directly hydrolyzes ATP to move 3 Na+ out and 2 K+ into the cell against their respective electrochemical gradients. * **Option D (Endocytosis):** This is a form of **Vesicular/Bulk Transport**. The formation of vesicles, movement of the cytoskeleton, and membrane pinching are active processes that require significant ATP consumption. **High-Yield Clinical Pearls for NEET-PG:** * **Aquaporins:** These are the "water channels." **AQP-2** is the specific channel in the renal collecting ducts regulated by ADH (Vasopressin). * **Gibbs-Donnan Effect:** Describes the behavior of charged particles near a semi-permeable membrane that sometimes fails to distribute evenly, influencing osmotic pressure. * **Solvent Drag:** A phenomenon where water moving by osmosis "drags" dissolved solutes along with it; this is also a passive process.

Question 257: Repolarization of a nerve action potential is due to which of the following?

A. Inward sodium current
B. Inward potassium current
C. Inward potassium current and outward sodium current
D. Outward potassium current and inward sodium current (Correct Answer)

Explanation: **Explanation:** The nerve action potential is a rapid change in membrane potential involving two main phases: depolarization and repolarization. **1. Why the Correct Answer is Right:** Repolarization is the process of returning the membrane potential to its negative resting state. This occurs due to two simultaneous events: * **Inactivation of Voltage-Gated Sodium Channels:** The "h-gates" (inactivation gates) close, stopping the **inward sodium current**. * **Activation of Voltage-Gated Potassium Channels:** These channels open slowly, allowing potassium ions to move down their electrochemical gradient out of the cell. This **outward potassium current** removes positive charges from the intracellular fluid, restoring negativity. *Note: While the question phrasing in some formats includes "inward sodium current," it refers to the cessation/inactivation of that current alongside the dominant outward potassium flow.* **2. Analysis of Incorrect Options:** * **Option A:** Inward sodium current is responsible for **Depolarization**, not repolarization. * **Option B & C:** Potassium movement during an action potential is almost exclusively **outward** because the intracellular concentration of $K^+$ is much higher than the extracellular concentration. An "inward potassium current" would further depolarize the cell. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Tetrodotoxin (Pufferfish) & Saxitoxin:** Block voltage-gated $Na^+$ channels, preventing depolarization. * **Tetraethylammonium (TEA):** Blocks voltage-gated $K^+$ channels, specifically inhibiting repolarization. * **Hyperkalemia:** Increases resting membrane potential (making it less negative), which initially increases excitability but eventually leads to inactivation of $Na^+$ channels (refractory state). * **After-hyperpolarization:** Caused by the slow closure of $K^+$ channels, allowing the potential to become more negative than the resting membrane potential (RMP).

Question 258: Which cell organelle lacks a membrane?

A. Mitochondria
B. Nucleus
C. Nucleolus (Correct Answer)
D. Endoplasmic reticulum

Explanation: **Explanation:** The **nucleolus** is the correct answer because it is a dense, non-membrane-bound structure located within the nucleoplasm. It is essentially a large aggregate of macromolecules—specifically ribosomal RNA (rRNA), proteins, and DNA (nucleolar organizer regions). Because it lacks a phospholipid bilayer, it is often described as a "biomolecular condensate" formed through liquid-liquid phase separation. Its primary function is the synthesis of rRNA and the assembly of ribosomal subunits. **Analysis of Incorrect Options:** * **Mitochondria (A):** These are double-membrane-bound organelles. The outer membrane serves as a protective barrier, while the inner membrane is folded into cristae to facilitate the electron transport chain. * **Nucleus (B):** The nucleus is enclosed by the nuclear envelope, a double-membrane structure (inner and outer nuclear membranes) perforated by nuclear pores. * **Endoplasmic Reticulum (D):** The ER is an extensive network of membrane-enclosed sacs (cisternae) and tubules. It is a single-membrane organelle continuous with the outer nuclear membrane. **High-Yield NEET-PG Pearls:** * **Membraneless Organelles:** Besides the nucleolus, other structures lacking a membrane include **ribosomes, centrioles/centrosomes, and the cytoskeleton** (microtubules, microfilaments). * **Double-Membrane Organelles:** Remember the mnemonic **M**any **N**ice **C**hloroplasts (**M**itochondria, **N**ucleus, **C**hloroplasts). * **Clinical Correlation:** The size and number of nucleoli increase in cells with high protein synthesis requirements, such as rapidly dividing cancer cells (a key feature in histopathology).

Question 259: Which of the following is not mediated through a negative feedback mechanism?

A. Blood Pressure (BP) regulation
B. Growth Hormone (GH) formation
C. Thrombus formation (Correct Answer)
D. Adrenocorticotropic Hormone (ACTH) release

Explanation: **Explanation:** In physiology, control systems are primarily governed by two mechanisms: **Negative Feedback** (which maintains homeostasis by reversing a deviation from a set point) and **Positive Feedback** (which amplifies a stimulus, leading to an "explosive" or "vicious cycle" effect). **Why Thrombus formation is the correct answer:** Thrombus formation (blood clotting) is a classic example of a **Positive Feedback Mechanism**. When a vessel is injured, platelets adhere to the site and release chemicals that attract more platelets. This cycle continues and amplifies until the "plug" is formed. Other examples of positive feedback include the LH surge during ovulation, the Ferguson reflex (oxytocin release during labor), and the depolarization phase of an action potential. **Why the other options are incorrect:** * **BP Regulation:** Regulated via the baroreceptor reflex. An increase in BP triggers mechanisms to decrease it, and vice versa, making it a negative feedback loop. * **GH Formation & ACTH Release:** Most endocrine axes operate on negative feedback. High levels of a peripheral hormone (like Cortisol or IGF-1) inhibit the release of their respective stimulating hormones (ACTH or GH) from the pituitary to maintain hormonal balance. **NEET-PG High-Yield Pearls:** * **Homeostasis:** Almost all physiological systems (Temperature, pH, Osmolarity) utilize negative feedback. * **Gain of Control System:** It is the measure of the effectiveness of a control system. It is calculated as **Gain = Correction / Residual Error**. Negative feedback systems have a high gain. * **Exception:** While most positive feedback is "unstable," the three physiological exceptions to remember for exams are **Clotting, Childbirth (Parturition), and the LH Surge.**

Question 260: Which cell junction allows the exchange of cytoplasmic molecules?

A. Gap junction (Correct Answer)
B. Tight junction
C. Anchoring junction
D. None of the above

Explanation: **Explanation:** **Gap junctions** (also known as nexus junctions) are specialized intercellular connections that directly link the cytoplasm of two cells. They are composed of transmembrane proteins called **connexins**, six of which assemble to form a hemichannel called a **connexon**. When connexons of adjacent cells align, they create a continuous aqueous channel. This allows the rapid, bidirectional exchange of ions (like $Ca^{2+}$), small metabolites, and second messengers (like cAMP), facilitating **electrical and metabolic coupling**. **Why other options are incorrect:** * **Tight Junctions (Zonula Occludens):** These act as a "barrier" or "seal" rather than a bridge. They fuse the outer layers of plasma membranes to prevent the paracellular leakage of water and solutes, maintaining cell polarity. * **Anchoring Junctions:** These provide mechanical stability. Examples include **Desmosomes** (cell-to-cell) and **Hemidesmosomes** (cell-to-matrix). They link the cytoskeletons of adjacent cells but do not allow the exchange of cytoplasmic molecules. **High-Yield Clinical Pearls for NEET-PG:** * **Cardiac Physiology:** Gap junctions are a key component of **intercalated discs**, allowing the heart to function as a functional syncytium for coordinated contraction. * **Smooth Muscle:** They are abundant in "unitary" (single-unit) smooth muscles, such as the uterus and GI tract. * **Clinical Correlation:** Mutations in connexin genes are linked to specific pathologies, such as **Connexin 26** mutations causing congenital non-syndromic deafness and **Connexin 32** mutations linked to X-linked Charcot-Marie-Tooth disease.

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