General Physiology — MCQs

General Physiology Indian Medical PG Practice Questions and MCQs

Question 281: Inner hair cells depolarize due to which of the following?

A. Potassium influx (Correct Answer)
B. Sodium influx
C. Potassium efflux
D. Voltage-gated calcium channels

Explanation: **Explanation:** The depolarization of inner hair cells (IHCs) is a unique physiological process that deviates from the standard sodium-driven depolarization seen in nerves and muscles. **1. Why Potassium Influx is Correct:** The apical portion of the hair cells, including the stereocilia, is bathed in **endolymph**, which is found in the scala media. Unlike typical extracellular fluid, endolymph is uniquely rich in **Potassium ($K^+$)** and has a high positive potential (+80 mV). When sound waves cause the stereocilia to bend toward the tallest kinocilium, mechanically-gated $K^+$ channels (MET channels) open. Due to the strong electrochemical gradient (the difference between the +80 mV endolymph and the -60 mV intracellular potential of the hair cell), **Potassium ions rush into the cell**, causing depolarization. **2. Why the Other Options are Incorrect:** * **Sodium Influx:** While $Na^+$ drives depolarization in most excitable tissues, the concentration of $Na^+$ in endolymph is extremely low. Therefore, it does not play a primary role in hair cell depolarization. * **Potassium Efflux:** In most cells, $K^+$ leaving the cell causes repolarization. In hair cells, $K^+$ efflux occurs at the **base** of the cell (into the perilymph) to *repolarize* the cell after the initial stimulus. * **Voltage-gated Calcium Channels:** These channels open **as a result** of the depolarization caused by $K^+$ influx. Their opening allows $Ca^{2+}$ to enter, triggering neurotransmitter (glutamate) release, but they are not the primary cause of the initial depolarization. **High-Yield NEET-PG Pearls:** * **Endolymph:** Produced by the **Stria Vascularis**; high $K^+$, low $Na^+$. * **Perilymph:** Similar to ECF/CSF; high $Na^+$, low $K^+$. * **Tip Links:** The structures connecting stereocilia that pull open the $K^+$ channels. * **Endocochlear Potential:** The +80 mV charge of the endolymph is the highest transepithelial potential in the body.

Question 282: What causes afterhyperpolarization?

A. Sodium efflux
B. Sodium influx
C. Chloride influx
D. Potassium efflux (Correct Answer)

Explanation: **Explanation:** **1. Why Potassium Efflux is Correct:** Afterhyperpolarization (also known as the undershoot phase) occurs at the end of an action potential. During repolarization, voltage-gated **K⁺ channels** open to allow potassium to leave the cell (**efflux**). However, these channels are slow to close. Even after the membrane potential reaches the resting level (-70 mV), K⁺ continues to exit the cell, moving the potential closer to the equilibrium potential of Potassium (approximately -90 mV). This transient period where the interior of the cell becomes more negative than the resting state is called afterhyperpolarization. **2. Why Other Options are Incorrect:** * **Sodium Influx (B):** This causes **depolarization** (the rising phase of the action potential) as positive ions enter the cell, making the interior less negative. * **Sodium Efflux (A):** While the Na⁺-K⁺ ATPase pump moves sodium out, it is a slow active transport process and not the primary driver of the rapid voltage changes seen in afterhyperpolarization. * **Chloride Influx (C):** While Cl⁻ entry can cause hyperpolarization (e.g., via GABA receptors), it is not the mechanism responsible for the "undershoot" following a standard neuronal action potential. **3. NEET-PG High-Yield Pearls:** * **Equilibrium Potentials:** Remember the Nernst values: K⁺ ≈ -94 mV, Na⁺ ≈ +61 mV. The membrane potential always moves toward the equilibrium potential of the ion to which it is most permeable. * **Refractory Period:** Afterhyperpolarization contributes to the **Relative Refractory Period**, meaning a stronger-than-normal stimulus is required to trigger another action potential. * **Channel Kinetics:** The key takeaway is that Na⁺ channels are "fast" (quick to open/inactivate), while K⁺ channels are "slow" (delayed opening/closing).

Question 283: What are the special transmembrane proteins that make up gap junctions called?

A. Cadherins
B. Occludins
C. Connexons (Correct Answer)
D. Claudins

Explanation: **Explanation:** **Gap junctions** are specialized intercellular connections that allow the direct passage of ions and small molecules between adjacent cells, facilitating electrical and metabolic coupling. 1. **Why Connexons are correct:** Each gap junction is composed of two hemichannels called **connexons** (one from each cell). Each connexon is formed by a hexameric assembly of six protein subunits known as **connexins**. When the connexons of two neighboring cells align, they form a continuous aqueous pore that connects the cytoplasm of both cells. This is crucial in tissues like the myocardium and smooth muscle for synchronized contraction. 2. **Why other options are incorrect:** * **Cadherins (A):** These are calcium-dependent adhesion molecules found in **Adherens junctions** and **Desmosomes**. They link the cytoskeleton of one cell to another but do not form channels. * **Occludins (B) and Claudins (D):** These are the primary transmembrane proteins that form **Tight Junctions (Zonula occludens)**. Their function is to seal the intercellular space to prevent the paracellular leakage of water and solutes, acting as a "barrier" rather than a "bridge." **Clinical Pearls & High-Yield Facts:** * **Connexin 43:** The most common connexin found in the heart. * **Charcot-Marie-Tooth disease (X-linked):** Caused by mutations in the Connexin 32 gene. * **Congenital Deafness:** Often associated with mutations in Connexin 26. * Gap junctions are the structural basis for **electrical synapses** in the CNS and the **intercalated discs** in cardiac muscle.

Question 284: Which of the following is not required for Polymerase Chain Reaction (PCR)?

A. Taq polymerase
B. Deoxynucleotide triphosphates (dNTPs)
C. Primer
D. Radiolabeled DNA probe (Correct Answer)

Explanation: **Explanation:** Polymerase Chain Reaction (PCR) is an *in vitro* technique used to amplify specific DNA sequences. The process mimics natural DNA replication but requires specific components to function in a thermal cycler. **Why Radiolabeled DNA probe is the correct answer:** A **DNA probe** is a single-stranded DNA fragment used to detect the presence of a complementary sequence within a sample (e.g., in Southern Blotting or *in situ* hybridization). PCR does not require a probe for the amplification process itself; it only requires primers to initiate synthesis. While some advanced variations like Real-Time PCR (qPCR) use fluorescent probes for detection, a standard PCR does not require them, and "radiolabeling" is specifically characteristic of older blotting techniques rather than PCR. **Analysis of Incorrect Options:** * **Taq polymerase:** This is a heat-stable DNA polymerase (derived from *Thermus aquaticus*) essential for synthesizing new DNA strands at high temperatures without denaturing. * **Deoxynucleotide triphosphates (dNTPs):** These (dATP, dCTP, dGTP, dTTP) are the fundamental building blocks (nucleotides) required to construct the new DNA strand. * **Primer:** PCR requires two synthetic, short oligonucleotide primers that are complementary to the regions flanking the target DNA to provide a 3'-OH group for polymerase extension. **High-Yield Clinical Pearls for NEET-PG:** * **Steps of PCR:** Denaturation (~94°C) → Annealing (~55°C) → Extension (~72°C). * **Southern Blot:** Used for DNA (uses probes). * **Northern Blot:** Used for RNA (uses probes). * **Western Blot:** Used for Proteins (uses antibodies). * **RT-PCR:** Uses Reverse Transcriptase to amplify RNA (essential for diagnosing viral infections like COVID-19).

Question 285: A 25-year-old female patient with a hemoglobin level of 4 gm/dL was given a blood transfusion with blood stored for 10 days. Which of the following statements is FALSE?

A. The oxygen-carrying capacity of this blood is reduced.
B. The 2,3-diphosphoglycerate (2,3-DPG) level falls in stored blood.
C. The fall in 2,3-DPG level is less when blood is stored in acid-citrate-dextrose solution. (Correct Answer)
D. An increase in 2,3-DPG levels causes a rightward shift of the oxygen-hemoglobin dissociation curve.

Explanation: ### Explanation This question tests the understanding of the **"Storage Lesion"** of blood and the role of **2,3-Diphosphoglycerate (2,3-DPG)** in oxygen kinetics. **Why Option C is the False Statement (Correct Answer):** When blood is stored, 2,3-DPG levels progressively decline because red cell glycolysis slows down. **Acid-Citrate-Dextrose (ACD)** solution actually causes a **more rapid and significant fall** in 2,3-DPG levels compared to newer preservatives like **CPD (Citrate-Phosphate-Dextrose)** or CPDA-1. The phosphate in CPD acts as a buffer, maintaining a higher pH which helps preserve 2,3-DPG levels for a longer duration. **Analysis of Other Options:** * **Option A:** While the physical amount of hemoglobin remains, the functional **oxygen-carrying capacity** (specifically the ability to release $O_2$ to tissues) is compromised due to the high oxygen affinity caused by low 2,3-DPG. * **Option B:** It is a physiological fact that 2,3-DPG levels fall during storage (reaching near-zero levels by 10–14 days in ACD). * **Option C:** As explained, ACD is inferior to CPD in preserving 2,3-DPG. * **Option D:** 2,3-DPG binds to the beta-chains of deoxyhemoglobin, stabilizing the "T" (Tense) state. An increase in 2,3-DPG decreases $O_2$ affinity, shifting the curve to the **Right**, facilitating $O_2$ unloading. **High-Yield Clinical Pearls for NEET-PG:** * **Left Shift (Increased Affinity):** $\downarrow$ 2,3-DPG, $\downarrow$ Temp, $\downarrow$ $H^+$ (Alkalosis), HbF, CO poisoning. * **Right Shift (Decreased Affinity):** $\uparrow$ 2,3-DPG, $\uparrow$ Temp, $\uparrow$ $H^+$ (Acidosis/Bohr Effect), $\uparrow$ $CO_2$. * **Storage Lesion:** Stored blood shows $\downarrow$ 2,3-DPG, $\downarrow$ pH, $\downarrow$ ATP, and **$\uparrow$ Potassium** (due to Na-K pump failure). * **Recovery:** After transfusion, it takes **6–24 hours** for the recipient's RBCs to regenerate 2,3-DPG levels to normal.

Question 286: Hyponatremia has no effect on the resting membrane potential (RMP) because:

A. The cell membrane is impermeable to Na+.
B. Extracellular fluid Na+ concentration is high.
C. Intracellular K+ concentration is high.
D. The equilibrium potential for Na+ is +60 mV. (Correct Answer)

Explanation: ### Explanation The **Resting Membrane Potential (RMP)** of a cell is primarily determined by the ion with the highest relative permeability at rest. In most excitable cells, the membrane is significantly more permeable to **Potassium ($K^+$)** than to **Sodium ($Na^+$)**. **1. Why Option D is Correct:** According to the **Goldman-Hodgkin-Katz equation**, the RMP is a weighted average of the equilibrium potentials of all permeant ions. The equilibrium potential for $Na^+$ ($E_{Na}$) is approximately **+60 mV**, while the RMP is typically **-70 to -90 mV** (close to the $E_K$ of -94 mV). Because the resting membrane conductance (permeability) for $Na^+$ is extremely low, changes in the extracellular $Na^+$ concentration (hyponatremia) have a negligible impact on the RMP. $Na^+$ primarily influences the **amplitude and peak of the action potential**, not the resting state. **2. Why the other options are incorrect:** * **Option A:** The membrane is not *completely* impermeable to $Na^+$; there is a small "leak," but it is 50–100 times less than the $K^+$ leak. * **Option B:** While ECF $Na^+$ is high (135–145 mEq/L), this fact alone doesn't explain why its *reduction* doesn't affect RMP. * **Option C:** High intracellular $K^+$ explains why $K^+$ is the primary determinant of RMP, but it doesn't directly explain the lack of $Na^+$ influence. ### High-Yield NEET-PG Pearls * **RMP Determinants:** The most important factor maintaining RMP is the **$K^+$ leak channels**. The **$Na^+$-$K^+$ ATPase pump** is electrogenic and contributes only about -4 to -10 mV directly. * **Hyperkalemia vs. Hyponatremia:** Changes in ECF $K^+$ (Hyper/Hypokalemia) significantly shift the RMP, leading to altered excitability (e.g., peaked T-waves in ECG). Changes in ECF $Na^+$ affect the **slope and height of the Action Potential upstroke**. * **Nernst Equation:** Used to calculate the equilibrium potential for a single ion. For $Na^+$, $E = +61 \log ([Na^+]_{out} / [Na^+]_{in})$.

Question 287: Intracellular receptors are found in which of the following classes of hormones?

A. Insulin
B. Glucagon
C. Corticosteroids (Correct Answer)
D. Growth hormone

Explanation: **Explanation:** The mechanism of hormone action is determined by the chemical nature of the hormone. Hormones are broadly classified into **water-soluble** (lipophobic) and **lipid-soluble** (lipophilic) molecules. **1. Why Corticosteroids are correct:** Corticosteroids (like cortisol and aldosterone) are steroid hormones derived from cholesterol. Being **lipophilic**, they easily cross the lipid bilayer of the cell membrane. Once inside, they bind to **intracellular receptors** (specifically cytoplasmic receptors for glucocorticoids or nuclear receptors for thyroid hormones). The hormone-receptor complex then translocates to the nucleus to act as a transcription factor, altering gene expression. **2. Why the other options are incorrect:** * **Insulin & Growth Hormone:** These are peptide/protein hormones. They are large and water-soluble, meaning they cannot cross the cell membrane. They bind to **enzyme-linked receptors** (Tyrosine Kinase for Insulin; JAK-STAT pathway for Growth Hormone) located on the cell surface. * **Glucagon:** This is a peptide hormone that binds to **G-Protein Coupled Receptors (GPCR)** on the cell surface, utilizing the cAMP second messenger system. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Intracellular Receptors:** "PET CAT" – **P**rogesterone, **E**strogen, **T**estosterone, **C**ortisol, **A**ldosterone, **T**hyroid hormones (T3/T4), and Vitamin D. * **Thyroid Hormones Exception:** Unlike most lipid-soluble hormones that bind in the cytoplasm, Thyroid hormone receptors are always located **directly on the chromatin** in the nucleus. * **Fast vs. Slow:** Surface receptor signaling (Insulin/Glucagon) is rapid (seconds to minutes), while intracellular receptor signaling (Steroids) is slow (hours to days) as it requires protein synthesis.

Question 288: What is the partial pressure of oxygen at an atmospheric pressure of 760 mm of Hg?

A. 76 mm of Hg
B. 160 mm of Hg (Correct Answer)
C. 120 mm of Hg
D. 140 mm of Hg

Explanation: **Explanation:** The partial pressure of a gas in a mixture is determined by **Dalton’s Law**, which states that the total pressure exerted by a mixture of gases is equal to the sum of the partial pressures of the individual gases. To calculate the partial pressure of oxygen ($PO_2$) in dry atmospheric air: 1. **Atmospheric Pressure ($P_{atm}$):** 760 mm Hg (at sea level). 2. **Fraction of Oxygen ($FiO_2$):** Oxygen constitutes approximately 21% of the atmospheric air. 3. **Calculation:** $PO_2 = P_{atm} \times FiO_2$ * $PO_2 = 760 \times 0.21 \approx \mathbf{159.6 \text{ mm Hg}}$ (rounded to **160 mm Hg**). **Analysis of Incorrect Options:** * **Option A (76 mm Hg):** This represents 10% of atmospheric pressure, which does not correspond to any physiological gas concentration at sea level. * **Option C (120 mm Hg):** This is closer to the $PO_2$ of **humidified tracheal air** (approx. 149–150 mm Hg) or represents a significant drop due to high altitude. * **Option D (140 mm Hg):** This value is lower than atmospheric $PO_2$ but higher than alveolar $PO_2$. **High-Yield Clinical Pearls for NEET-PG:** * **Humidified Air:** Once air enters the upper airways, it is saturated with water vapor ($PH_2O = 47 \text{ mm Hg}$). The $PO_2$ drops to: $(760 - 47) \times 0.21 \approx \mathbf{150 \text{ mm Hg}}$. * **Alveolar Air ($PAO_2$):** Due to the continuous uptake of $O_2$ and the addition of $CO_2$, the $PO_2$ in the alveoli is approximately **100–104 mm Hg**. * **Fractional Concentration:** Note that while $PO_2$ decreases with increasing altitude (as $P_{atm}$ drops), the **percentage** of oxygen in the air remains constant at 21%.

Question 289: Maximum number of Na+ channels per square micrometer is present in which part of the neuron?

A. Cell body
B. Axon terminal
C. Surface of myelin
D. Nodes of Ranvier (Correct Answer)

Explanation: **Explanation:** The density of voltage-gated sodium (Na+) channels is the primary determinant of the threshold for excitation and the velocity of impulse conduction. **Why Nodes of Ranvier is correct:** In myelinated neurons, the axonal membrane is exposed to the extracellular fluid only at the **Nodes of Ranvier**. To facilitate **saltatory conduction**, Na+ channels are highly concentrated at these gaps (approximately **2,000–12,000 per µm²**). This high density ensures that the depolarization is strong enough to "jump" from one node to the next, significantly increasing conduction velocity compared to unmyelinated fibers. **Why the other options are incorrect:** * **Cell body (Soma):** Contains a relatively low density of Na+ channels (approx. 50–75 per µm²), as its primary role is metabolic support and signal integration rather than rapid action potential propagation. * **Axon terminal:** While it contains Na+ channels and voltage-gated Ca²⁺ channels for neurotransmitter release, the density does not reach the levels found at the nodes. * **Surface of myelin:** Myelin is an insulating fatty layer produced by Schwann cells (PNS) or Oligodendrocytes (CNS). It is virtually devoid of ion channels; its function is to increase electrical resistance and decrease capacitance. **High-Yield NEET-PG Pearls:** 1. **Axon Hillock:** This is the site of action potential **initiation** because it has the lowest threshold for excitation (highest density of Na+ channels *outside* of the nodes). 2. **Demyelinating Diseases:** In conditions like **Multiple Sclerosis (CNS)** or **Guillain-Barré Syndrome (PNS)**, the loss of myelin exposes segments with low Na+ channel density, leading to conduction block or slowing. 3. **Internodes:** The regions under the myelin sheath have very few Na+ channels.

Question 290: Which of the following major minerals is a component of biological membranes?

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

Explanation: **Explanation:** **Correct Option: C (Phosphorus)** The fundamental structural unit of all biological membranes (cell membranes and organelle membranes) is the **phospholipid bilayer**. Phospholipids are amphipathic molecules consisting of a glycerol backbone, two fatty acid tails, and a **phosphate group** in the hydrophilic head. Therefore, phosphorus is an essential structural component of the membrane matrix. Additionally, phosphorus is vital for energy metabolism (ATP), signal transduction (phosphorylation), and the structure of nucleic acids (DNA/RNA). **Why other options are incorrect:** * **A. Calcium:** While calcium is crucial for stabilizing the membrane structure and regulating membrane permeability (by binding to phospholipids), it is not a primary structural constituent of the lipid bilayer itself. It functions more as a signaling ion and a component of the extracellular matrix/bone. * **B. Sodium & D. Potassium:** These are the primary electrolytes responsible for maintaining the resting membrane potential and osmotic balance. Sodium is the major extracellular cation, and Potassium is the major intracellular cation. While they interact with membrane proteins (like the Na+/K+ ATPase), they are not structural components of the membrane. **High-Yield NEET-PG Pearls:** * **Phospholipid Composition:** The most abundant phospholipid in the cell membrane is **Phosphatidylcholine** (Lecithin). * **Asymmetry:** Phosphatidylserine is normally sequestered in the inner leaflet; its appearance on the outer leaflet is a hallmark of **apoptosis** (recognized by macrophages). * **Fluidity:** Membrane fluidity is determined by the ratio of saturated to unsaturated fatty acids and the presence of **cholesterol**. * **Phosphorus Homeostasis:** It is primarily regulated by the kidneys under the influence of PTH (which is phosphaturic) and FGF-23.

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