General Physiology — MCQs

General Physiology Indian Medical PG Practice Questions and MCQs

Question 21: Presynaptic facilitation is caused by?

A. Prolonged opening of calcium channels (Correct Answer)
B. Prolonged opening of chloride channels
C. Prolonged opening of sodium channels
D. Prolonged closure of potassium channels

Explanation: **Explanation:** **Presynaptic facilitation** is a mechanism where the amount of neurotransmitter released from a presynaptic terminal is increased. This occurs when an interneuron (facilitator neuron) synapses onto the presynaptic terminal of a sensory neuron, a process known as an **axoxonal synapse**. 1. **Why Option A is correct:** The facilitator neuron releases serotonin, which increases intracellular cAMP levels in the sensory terminal. This leads to the phosphorylation and **closure of potassium (K+) channels**. When K+ channels are closed, repolarization is delayed, leading to a **prolonged action potential**. This extended depolarization keeps voltage-gated **calcium (Ca2+) channels open for a longer duration**. The resulting influx of Ca2+ triggers the exocytosis of a greater number of neurotransmitter vesicles, thereby facilitating synaptic transmission. 2. **Why other options are incorrect:** * **Option B:** Opening of chloride channels typically causes hyperpolarization (Inhibitory Postsynaptic Potential - IPSP), leading to presynaptic inhibition, not facilitation. * **Option C:** While sodium channels initiate the action potential, facilitation specifically depends on the *duration* of the plateau phase maintained by calcium. * **Option D:** While the *closure* of K+ channels is the initiating step, the **direct cause** of increased neurotransmitter release is the **prolonged opening of calcium channels**. In medical exams, the final common pathway (Ca2+ influx) is the preferred answer. **High-Yield Facts for NEET-PG:** * **Molecular Basis:** Presynaptic facilitation is the physiological basis for **Sensitization** (a form of non-associative learning). * **Neurotransmitter involved:** Serotonin (5-HT) is the primary mediator in the *Aplysia* model used to study this phenomenon. * **Presynaptic Inhibition:** Conversely, this is caused by a *decrease* in Ca2+ influx, often mediated by GABA which increases Cl- conductance or decreases Ca2+ channel opening.

Question 22: In apoptosis, permeabilization of which membrane occurs?

A. Nuclear membrane
B. Cytoplasmic membrane
C. Lysosome
D. Mitochondrial membrane (Correct Answer)

Explanation: ### Explanation **Correct Option: D. Mitochondrial Membrane** Apoptosis (programmed cell death) is primarily regulated through the **intrinsic (mitochondrial) pathway**. The central event in this pathway is the **Mitochondrial Outer Membrane Permeabilization (MOMP)**. This process is controlled by the Bcl-2 family of proteins. Pro-apoptotic proteins like **BAX and BAK** create pores in the outer mitochondrial membrane, leading to the leakage of **Cytochrome c** into the cytosol. Once in the cytosol, Cytochrome c binds with APAF-1 to form the **apoptosome**, which activates Caspase-9, eventually leading to cell death. **Why other options are incorrect:** * **A. Nuclear membrane:** While nuclear changes like chromatin condensation (pyknosis) and fragmentation (karyorrhexis) occur during apoptosis, the permeabilization of the nuclear membrane is not the initiating or defining event. * **B. Cytoplasmic membrane:** In apoptosis, the plasma membrane remains **intact** (though it undergoes "blebbing") to prevent the leakage of cellular contents and subsequent inflammation. This is a key distinction from **necrosis**, where the plasma membrane is ruptured. * **C. Lysosome:** Lysosomal membrane rupture is typically associated with autolysis or necrotic cell death. In apoptosis, organelles are generally sequestered into apoptotic bodies rather than being lysed. **High-Yield Clinical Pearls for NEET-PG:** * **Anti-apoptotic proteins:** Bcl-2, Bcl-xL (they maintain membrane stability). * **Pro-apoptotic proteins:** BAX, BAK (the "executioners" that cause permeabilization). * **BH3-only proteins:** BIM, BID, BAD (the "sensors" that initiate the pathway). * **Morphological Hallmark:** Apoptosis is characterized by cell shrinkage and intact membranes, whereas necrosis involves cell swelling and membrane rupture. * **Biochemical Hallmark:** DNA laddering (due to internucleosomal cleavage by endonucleases).

Question 23: Stem cells are primarily taken from which source?

A. Skin
B. Bone marrow (Correct Answer)
C. Oral mucosa
D. Elementary tract

Explanation: **Explanation:** **Correct Option: B (Bone marrow)** Bone marrow is the primary and most well-established source of adult stem cells, specifically **Hematopoietic Stem Cells (HSCs)** and **Mesenchymal Stem Cells (MSCs)**. HSCs are multipotent cells responsible for the continuous production of all blood cell lineages (erythrocytes, leukocytes, and platelets). Because the bone marrow maintains a high rate of cellular turnover throughout life, it serves as the richest reservoir for harvesting stem cells for clinical procedures like bone marrow transplants to treat leukemias and lymphomas. **Incorrect Options:** * **A (Skin):** While the skin contains epidermal stem cells (in the basal layer and hair follicle bulge), they are unipotent or bipotent, primarily functioning only for skin regeneration. They are not the "primary" source for systemic stem cell therapy. * **C & D (Oral mucosa & Alimentary tract):** These tissues contain rapidly dividing epithelial stem cells to replace the lining; however, they are difficult to isolate in significant quantities and lack the clinical versatility (multipotency) of bone marrow-derived cells. **NEET-PG High-Yield Pearls:** * **Potency Hierarchy:** Zygote is **Totipotent**; Embryonic stem cells (from the inner cell mass) are **Pluripotent**; Bone marrow HSCs are **Multipotent**. * **Markers:** CD34+ is the classic surface marker used to identify and isolate Hematopoietic Stem Cells. * **Alternative Sources:** Apart from bone marrow, stem cells can also be harvested from **Peripheral Blood** (after mobilization with G-CSF) and **Umbilical Cord Blood**.

Question 24: Which of the following statements is true regarding intracellular receptors?

A. They are mainly located on the nuclear surface. (Correct Answer)
B. Estrogen does not act on them.
C. Growth hormone (GH) acts on intracellular receptors.
D. Vitamin E acts on intracellular receptors.

Explanation: **Explanation:** **1. Why Option A is Correct:** Intracellular receptors are protein molecules located inside the target cell rather than on the cell membrane. These receptors primarily function as **ligand-activated transcription factors**. While some receptors (like glucocorticoids) are found in the cytoplasm, the majority—including those for thyroid hormones and many steroid hormones—are located **within the nucleus** or on the **nuclear surface**. Once the lipid-soluble ligand binds to the receptor, the complex binds to specific DNA sequences (Hormone Response Elements) to regulate gene transcription. **2. Why Other Options are Incorrect:** * **Option B:** Estrogen is a steroid hormone. Being lipophilic, it easily crosses the lipid bilayer and acts specifically on **intracellular (nuclear) receptors** (ER-α and ER-β). * **Option C:** Growth Hormone (GH) is a peptide hormone. Peptide hormones are water-soluble and cannot cross the cell membrane; therefore, GH acts via **cell surface receptors** (specifically the JAK-STAT signaling pathway). * **Option D:** While Vitamin A and Vitamin D act on intracellular receptors to regulate gene expression, **Vitamin E** (Tocopherol) primarily functions as a potent antioxidant within membranes and does not have a classic intracellular receptor-mediated signaling pathway like steroid hormones. **3. High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Intracellular Receptors:** "PET TV" – **P**rogesterone, **E**strogen, **T**estosterone, **T**hyroid hormone (T3/T4), and **V**itamin D/A. * **Thyroid Hormone Exception:** Unlike most others, Thyroid receptors are *always* bound to DNA in the nucleus, even in the absence of the hormone. * **Speed of Action:** Intracellular receptor signaling is **slow** (hours to days) because it requires protein synthesis, unlike ionotropic receptors which act in milliseconds.

Question 25: Vitamin K is essential for the gamma-carboxylation of which amino acid residue?

A. Glutamate (Correct Answer)
B. Aspartate
C. Glycine
D. Alanine

Explanation: **Explanation:** **Vitamin K** acts as a vital co-factor for the enzyme **gamma-glutamyl carboxylase**. This enzyme facilitates the post-translational modification of specific **Glutamate (Glu)** residues into **gamma-carboxyglutamate (Gla)**. This process adds a second carboxyl group to the glutamate side chain, creating a high-affinity binding site for calcium ions ($Ca^{2+}$). This "calcium bridge" is essential for the binding of clotting factors to phospholipid surfaces on platelets, thereby initiating the coagulation cascade. * **Why Glutamate is Correct:** Only glutamate residues possess the specific structure required for gamma-carboxylation. This modification is essential for Clotting Factors **II, VII, IX, and X**, as well as anticoagulant **Proteins C and S**. * **Why Incorrect Options are Wrong:** * **Aspartate:** While chemically similar to glutamate, it lacks the specific carbon chain length required by the carboxylase enzyme. * **Glycine & Alanine:** These are simple amino acids that do not possess the acidic side chains necessary for carboxylation or calcium binding. **High-Yield Clinical Pearls for NEET-PG:** * **Warfarin Mechanism:** Warfarin inhibits **Vitamin K Epoxide Reductase (VKOR)**, preventing the recycling of Vitamin K and thus inhibiting the gamma-carboxylation of glutamate. * **Osteocalcin:** Vitamin K is also required for the gamma-carboxylation of osteocalcin in bones, making it important for bone mineralization. * **Newborns:** They are Vitamin K deficient due to sterile guts and poor placental transfer; hence, a prophylactic Vitamin K injection is given at birth to prevent **Hemorrhagic Disease of the Newborn**.

Question 26: The inverse stretch reflex is due to which of the following?

A. Trail fibre ending
B. Golgi tendon organ (Correct Answer)
C. Tail fiber ending
D. Muscle spindle

Explanation: **Explanation:** The **Inverse Stretch Reflex** (also known as the autogenic inhibition reflex) is a protective mechanism that prevents muscle damage during excessive contraction. **1. Why Golgi Tendon Organ (GTO) is correct:** The GTO is a high-threshold encapsulated sensory receptor located at the **junction of muscle fibers and tendons**. Unlike the muscle spindle, which responds to changes in length, the GTO is arranged in **series** with muscle fibers and responds primarily to **muscle tension**. When a muscle contracts forcefully, the GTO is stimulated and sends impulses via **Ib afferent nerve fibers** to the spinal cord. These fibers synapse with inhibitory interneurons that inhibit the alpha motor neurons of the same muscle, causing it to relax. This prevents potential avulsion or tendon rupture. **2. Why other options are incorrect:** * **Muscle Spindle:** These are receptors arranged in **parallel** with extrafusal fibers. They detect changes in muscle **length** (stretch) and mediate the **Stretch Reflex** (e.g., knee jerk), which causes contraction, not relaxation. * **Trail fiber ending:** This refers to a type of gamma motor neuron ending on static nuclear bag or chain fibers within the muscle spindle. It is an efferent (motor) component, not the primary sensory mediator of the inverse reflex. * **Tail fiber ending:** This is a distractor term and does not correspond to a recognized physiological receptor in the neuromuscular system. **Clinical Pearls for NEET-PG:** * **Stretch Reflex:** Receptor = Muscle Spindle; Afferent = Ia; Result = Contraction. * **Inverse Stretch Reflex:** Receptor = GTO; Afferent = Ib; Result = Relaxation. * **Clasp-knife response:** Seen in upper motor neuron (UMN) lesions, this phenomenon is clinically attributed to the activation of the inverse stretch reflex when a spastic muscle is forcefully stretched.

Question 27: Which of the following transport processes is passive?

A. Co-transport of Na+ ion and a molecule into the cell
B. Exchange of K+ ion outside the cell for Na+ ion inside the cell
C. Flow of H2O through the cell membrane by osmosis (Correct Answer)
D. Uptake of molecules into the cell by endocytosis

Explanation: ### Explanation **Correct Option: C. Flow of H2O through the cell membrane by osmosis** **Why it is correct:** Passive transport refers to the movement of substances across a cell membrane without the expenditure of metabolic energy (ATP). **Osmosis** is the net diffusion of water across a selectively permeable membrane from a region of low solute concentration (high water potential) to a region of high solute concentration (low water potential). Since it occurs down a concentration gradient and does not require energy, it is a classic example of passive transport. **Analysis of Incorrect Options:** * **A. Co-transport of Na+ and a molecule:** This is **Secondary Active Transport** (specifically Symport). It utilizes the energy stored in the electrochemical gradient of sodium (created by the Na+-K+ ATPase) to move another molecule against its concentration gradient. * **B. Exchange of K+ for Na+:** This refers to the **Na+-K+ ATPase pump**, which is the hallmark of **Primary Active Transport**. It directly hydrolyzes ATP to move 3 Na+ out and 2 K+ into the cell against their respective gradients. * **D. Endocytosis:** This is a form of **Vesicular/Bulk Transport**. It is an active process requiring significant energy and cytoskeleton remodeling to engulf extracellular material. **High-Yield Clinical Pearls for NEET-PG:** * **Aquaporins:** While water can diffuse through the lipid bilayer, rapid movement in kidneys (collecting ducts) and RBCs occurs via specialized channels called **Aquaporins**. * **Gibbs-Donnan Effect:** Describes the behavior of charged particles near a semi-permeable membrane that sometimes fails to distribute evenly due to non-diffusible ions (like proteins), influencing osmotic pressure. * **SGLT-1/SGLT-2:** These are clinically significant examples of **Secondary Active Transport** (Co-transport) targeted by drugs like Gliflozins in Diabetes mellitus.

Question 28: Nitrogen narcosis is caused due to

A. Nitrogen inhibits dismutase enzyme
B. Increase in production of nitrous oxide
C. Increased solubility of nitrogen in nerve cell membrane (Correct Answer)
D. Decrease in oxygen free radicals

Explanation: ### Explanation **Nitrogen Narcosis** (also known as "Rapture of the Deep") occurs in deep-sea divers breathing compressed air at depths typically exceeding 100 feet (approx. 4 atmospheres of pressure). **1. Why the Correct Answer is Right:** The underlying mechanism is explained by the **Meyer-Overton Hypothesis**. Nitrogen is a chemically inert gas, but it is highly **lipid-soluble**. As a diver descends, the increasing partial pressure of nitrogen forces more gas to dissolve into the body tissues. Because of its high lipid solubility, nitrogen dissolves preferentially into the **lipid bilayer of nerve cell membranes**. This physical presence of nitrogen molecules interferes with ionic conductance across the neuronal membrane, acting similarly to volatile anesthetics. This leads to a progressive depression of the Central Nervous System (CNS), causing symptoms ranging from euphoria and impaired judgment to unconsciousness. **2. Why the Incorrect Options are Wrong:** * **Option A:** Nitrogen narcosis is a physical phenomenon based on solubility; it does not involve the inhibition of the dismutase enzyme. * **Option B:** Nitrous oxide ($N_2O$) is a different gas used in anesthesia. While nitrogen and nitrous oxide have similar anesthetic properties, nitrogen narcosis is caused by molecular nitrogen ($N_2$), not the production of $N_2O$. * **Option C:** Oxygen free radicals are associated with **Oxygen Toxicity** (Paul Bert effect), which can cause seizures at high pressures, but they are not the cause of nitrogen narcosis. **3. High-Yield Clinical Pearls for NEET-PG:** * **Martini’s Law:** A common rule of thumb stating that every 50 feet of depth is equivalent to drinking one glass of Martini in terms of intoxicating effect. * **Prevention:** Divers avoid narcosis by using **Heliox** (Helium + Oxygen) because Helium has much lower lipid solubility and lower density, reducing the narcotic effect and work of breathing. * **Decompression Sickness (The Bends):** Do not confuse narcosis with "The Bends." Narcosis happens *at depth* due to solubility; The Bends happens during *ascent* due to nitrogen bubble formation in tissues.

Question 29: According to Fick's first law of diffusion, the rate of simple diffusion is NOT proportional to which of the following?

A. Surface area
B. Temperature (Correct Answer)
C. Concentration gradient
D. Thickness of the membrane

Explanation: **Explanation:** Fick’s First Law of Diffusion describes the rate at which a substance moves across a biological membrane. The mathematical expression for the law is: **$J = -DA \frac{\Delta C}{\Delta X}$** *(Where $J$ = Rate of diffusion, $D$ = Diffusion coefficient, $A$ = Surface area, $\Delta C$ = Concentration gradient, and $\Delta X$ = Thickness of the membrane).* 1. **Why Temperature is the Correct Answer:** While temperature does influence the kinetic energy of molecules and affects the diffusion coefficient ($D$), it is **not a direct variable** in Fick’s First Law equation. In biological systems (human body), temperature is relatively constant ($37^\circ C$); therefore, the rate of diffusion is functionally independent of temperature fluctuations in a clinical context. 2. **Analysis of Incorrect Options:** * **Surface Area (A):** The rate is **directly proportional** to the surface area. For example, the extensive branching of alveoli increases surface area to maximize gas exchange. * **Concentration Gradient ($\Delta C$):** The rate is **directly proportional** to the difference in concentration between two sides. A steeper gradient results in faster diffusion. * **Thickness of Membrane ($\Delta X$):** The rate is **inversely proportional** to the thickness. This is why respiratory membranes are extremely thin (0.6 $\mu m$). **Clinical Pearls for NEET-PG:** * **Emphysema:** Reduces diffusion rate by decreasing the **Surface Area** (destruction of alveolar walls). * **Pulmonary Edema/Fibrosis:** Reduces diffusion rate by increasing the **Thickness** of the membrane. * **Graham’s Law:** Diffusion rate is inversely proportional to the square root of the molecular weight ($\sqrt{MW}$). This explains why $CO_2$ (heavier but more soluble) diffuses 20 times faster than $O_2$.

Question 30: What is the normal range of serum osmolarity?

A. 350-375 mOsm/kg
B. 200-250 mOsm/kg
C. 270-285 mOsm/kg (Correct Answer)
D. 300-320 mOsm/kg

Explanation: **Explanation:** The correct answer is **C (270-285 mOsm/kg)**. Serum osmolarity (or more accurately, osmolality) refers to the concentration of particles dissolved in the blood plasma. In a healthy adult, the body strictly maintains this range through the action of Antidiuretic Hormone (ADH) and the thirst mechanism, regulated by osmoreceptors in the hypothalamus. **Why C is correct:** Standard medical textbooks (like Guyton and Ganong) define the normal plasma osmolality range as **270–285 mOsm/kg** (some sources extend this to 295 mOsm/kg). Sodium ($Na^+$), along with its associated anions (Chloride and Bicarbonate), accounts for nearly 90% of this value. Glucose and Urea contribute the remainder. **Why other options are incorrect:** * **A & D (300-375 mOsm/kg):** These values represent **Hyperosmolar states**. Such levels are seen in severe dehydration, Diabetes Insipidus, or Hyperglycemic Hyperosmolar State (HHS). * **B (200-250 mOsm/kg):** These values represent **Hypoosmolar states**, typically seen in overhydration or SIADH (Syndrome of Inappropriate Antidiuretic Hormone). **High-Yield Clinical Pearls for NEET-PG:** 1. **Calculated Osmolarity Formula:** $2 \times [Na^+] + \frac{[Glucose]}{18} + \frac{[BUN]}{2.8}$. 2. **Osmolar Gap:** The difference between measured and calculated osmolarity. A gap **>10 mOsm/L** suggests the presence of unmeasured toxins like Ethanol, Methanol, or Ethylene glycol. 3. **Tight Regulation:** A mere **1-2% change** in osmolarity is sufficient to trigger ADH release or the thirst sensation. 4. **Effective Osmoles:** Sodium and Glucose are "effective osmoles" because they do not freely cross cell membranes and thus exert osmotic pressure. Urea is an "ineffective osmole" as it crosses membranes freely.

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