General Physiology — MCQs

General Physiology Indian Medical PG Practice Questions and MCQs

Question 271: When the sympathetic nervous system is activated, what occurs?

A. Norepinephrine is released by the vascular smooth muscle cells
B. Acetylcholine is released onto vascular smooth muscle cells
C. Norepinephrine is released from axons onto the arteriolar wall (Correct Answer)
D. The arterioles constrict because nitric oxide production is suppressed

Explanation: ### Explanation **1. Why the Correct Answer is Right:** The sympathetic nervous system (SNS) regulates vascular tone primarily through **postganglionic sympathetic adrenergic fibers**. When the SNS is activated, these axons release **Norepinephrine (NE)** from their nerve terminals (varicosities) directly onto the smooth muscle cells of the arteriolar wall. NE then binds to **$\alpha_1$-adrenergic receptors**, triggering a G-protein-coupled signaling cascade that leads to vasoconstriction. This is a fundamental mechanism for maintaining peripheral resistance and systemic blood pressure. **2. Why the Incorrect Options are Wrong:** * **Option A:** Norepinephrine is a neurotransmitter synthesized and released by **neurons** (specifically postganglionic sympathetic fibers), not by the vascular smooth muscle cells themselves. The muscle cells are the *targets*, not the source. * **Option B:** Acetylcholine (ACh) is the neurotransmitter for the parasympathetic nervous system and preganglionic sympathetic fibers. While some specialized sympathetic fibers (to sweat glands) release ACh, the primary sympathetic control of arterioles is **adrenergic (NE)**, not cholinergic. * **Option D:** While SNS activation causes constriction, it is mediated by the direct action of NE on $\alpha_1$ receptors. Nitric oxide (NO) is a potent vasodilator produced by the endothelium; while its inhibition causes constriction, the *primary* mechanism of SNS-induced constriction is the release of NE, not the suppression of NO. **3. High-Yield NEET-PG Pearls:** * **Exceptions to the Rule:** Most sympathetic postganglionic neurons release NE, but those innervating **sweat glands** release **Acetylcholine** (Sympathetic Cholinergic). * **Receptor Specificity:** $\alpha_1$ receptors cause vasoconstriction (skin, viscera), while $\beta_2$ receptors (activated primarily by circulating Epinephrine) cause vasodilation in skeletal muscle and coronary arteries. * **Adrenal Medulla:** Unlike other sympathetic pathways, the adrenal medulla is innervated by **preganglionic** fibers and releases Epinephrine (80%) and NE (20%) directly into the blood.

Question 272: The characteristic feature of apoptosis on light microscopy is?

A. Cellular swelling
B. Nuclear compaction (Correct Answer)
C. Intact cell membrane
D. Cytoplasmic eosinophilia

Explanation: **Explanation:** Apoptosis, or programmed cell death, is a highly regulated process characterized by specific morphological changes. The hallmark feature visible under light microscopy is **Nuclear Compaction** (pyknosis). This occurs due to the condensation of chromatin into dense, well-delimited masses that aggregate under the nuclear membrane. This is often followed by **karyorrhexis** (nuclear fragmentation). **Analysis of Options:** * **A. Cellular Swelling:** This is a feature of **Necrosis** (oncosis), resulting from the failure of ATP-dependent ion pumps. In contrast, apoptosis involves **cellular shrinkage**, where the cell becomes smaller and the cytoplasm becomes dense. * **C. Intact Cell Membrane:** While it is true that the plasma membrane remains structurally intact during the early stages of apoptosis (preventing inflammation), this is a **structural** feature rather than a diagnostic "characteristic feature" used to identify the process under light microscopy. Furthermore, the membrane eventually undergoes "blebbing" to form apoptotic bodies. * **D. Cytoplasmic Eosinophilia:** While apoptotic cells do show increased eosinophilia (pinker appearance) due to the loss of cytoplasmic RNA and protein denaturation, this is a non-specific finding also seen in early necrosis. Nuclear changes are the most definitive diagnostic features of apoptosis. **High-Yield Pearls for NEET-PG:** * **Gold Standard for Detection:** DNA Laddering (Step-ladder pattern on gel electrophoresis) due to internucleosomal cleavage by Ca²⁺/Mg²⁺ dependent endonucleases. * **Most Characteristic Feature (EM):** Chromatin condensation. * **Marker for Apoptotic Cells:** Presence of **Phosphatidylserine** on the outer leaflet of the plasma membrane (detected by Annexin V). * **Caspases:** The executioners of apoptosis (Caspase 3 is the common executioner).

Question 273: Which type of sensory receptor provides information about the force of muscle contraction?

A. Nuclear bag fiber
B. Nuclear chain fiber
C. Golgi tendon organ (Correct Answer)
D. Bare nerve ending

Explanation: ### Explanation The correct answer is **Golgi tendon organ (GTO)**. **1. Why Golgi Tendon Organ is Correct:** The Golgi tendon organ is a specialized sensory receptor located in the muscle tendons, arranged **in series** with the extrafusal muscle fibers. Its primary function is to sense **muscle tension** (force). When a muscle contracts, it pulls on the tendon, stimulating the GTO. This information is transmitted via **Type Ib afferent fibers** to the spinal cord, where it triggers the **inverse stretch reflex** (autogenic inhibition), causing the muscle to relax to prevent injury from excessive force. **2. Why the Other Options are Incorrect:** * **Options A & B (Nuclear Bag and Chain Fibers):** These are types of **intrafusal fibers** located within the **Muscle Spindle**. Muscle spindles are arranged **in parallel** with extrafusal fibers and are sensitive to changes in **muscle length** (stretch) rather than force. * *Nuclear bag fibers* primarily detect dynamic changes (velocity of stretch). * *Nuclear chain fibers* primarily detect static changes (constant length). * **Option D (Bare Nerve Ending):** These are primarily **nociceptors** (pain) or **thermoreceptors** (temperature). They do not provide specific feedback regarding the mechanical force of muscle contraction. **3. High-Yield Clinical Pearls for NEET-PG:** * **Muscle Spindle = Length** (Type Ia and II fibers); **GTO = Tension/Force** (Type Ib fibers). * **Arrangement:** Spindles are "In Parallel"; GTOs are "In Series." * **Reflexes:** Muscle spindle activation leads to contraction (Stretch Reflex); GTO activation leads to relaxation (Inverse Stretch Reflex). * **Protopathic Sensation:** Crude touch and pressure are carried by the anterior spinothalamic tract, whereas proprioception (position/force) is carried by the dorsal column-medial lemniscus pathway.

Question 274: What is the most abundant extracellular buffer in the human body?

A. Haemoglobin
B. Plasma proteins
C. Bicarbonate (Correct Answer)
D. Phosphate

Explanation: **Explanation:** The acid-base balance of the body is maintained by various buffer systems. The **Bicarbonate buffer system ($HCO_3^– / CO_2$)** is the most abundant and important buffer in the **extracellular fluid (ECF)**. Its primary strength lies in the fact that it is an "open system": the concentration of $HCO_3^–$ is regulated by the kidneys, while $CO_2$ levels are controlled by the lungs (respiration). This allows the body to rapidly compensate for pH changes. **Analysis of Options:** * **A. Haemoglobin:** This is a potent buffer, but it is located exclusively **intracellularly** (within Red Blood Cells). It is crucial for buffering $H^+$ ions generated during $CO_2$ transport (Bohr effect). * **B. Plasma Proteins:** These act as extracellular buffers due to their amino acid side chains, but their concentration and buffering capacity are significantly lower than the bicarbonate system. * **D. Phosphate:** The phosphate buffer system is highly efficient due to its pKa (6.8) being close to physiological pH. However, it is the **most abundant intracellular buffer** and is also vital in renal tubular fluid. Its concentration in the ECF is too low to be the primary buffer there. **High-Yield Clinical Pearls for NEET-PG:** * **Henderson-Hasselbalch Equation:** $pH = pKa + \log ([HCO_3^-] / [0.03 \times PCO_2])$. * **First line of defense:** Chemical buffers (seconds). * **Second line:** Respiratory system (minutes). * **Third line:** Renal system (hours to days; most powerful). * **Isohydric Principle:** All buffer systems in a common solution are in equilibrium; a change in one affects all others.

Question 275: The agranular endoplasmic reticulum is primarily involved in the synthesis of which of the following substances?

A. Protein
B. Lipid (Correct Answer)
C. Carbohydrate
D. Vitamin D

Explanation: **Explanation:** The **Smooth Endoplasmic Reticulum (SER)**, also known as the **agranular endoplasmic reticulum**, is characterized by the absence of ribosomes on its surface. Its primary physiological role is the **synthesis of lipids**, including phospholipids and cholesterol, which are essential for membrane formation. In specialized cells, the SER is also the site for the synthesis of steroid hormones (e.g., in the adrenal cortex and gonads). **Analysis of Options:** * **Option A (Protein):** This is incorrect. Protein synthesis is the primary function of the **Rough Endoplasmic Reticulum (RER)**, which is "granular" due to the presence of attached ribosomes. * **Option C (Carbohydrate):** While the SER is involved in glycogenolysis (specifically via the enzyme *Glucose-6-phosphatase*), it is not the primary site for carbohydrate synthesis. * **Option D (Vitamin D):** Vitamin D synthesis begins in the skin via UV light and involves hydroxylation in the liver and kidneys (mitochondria and microsomes), but it is not the primary defining function of the agranular ER. **High-Yield NEET-PG Pearls:** 1. **Detoxification:** The SER in hepatocytes contains the **Cytochrome P450** system, which is crucial for the detoxification of drugs and toxins. 2. **Sarcoplasmic Reticulum:** In skeletal and cardiac muscle, a specialized form of SER (Sarcoplasmic Reticulum) acts as the primary storage site for **Calcium ions ($Ca^{2+}$)**, essential for muscle contraction. 3. **Organelle Marker:** *Glucose-6-phosphatase* is a classic biochemical marker for the Endoplasmic Reticulum. 4. **Nissl Bodies:** In neurons, the RER is referred to as Nissl bodies; note that these are absent in the axon and axon hillock.

Question 276: What is the barometric pressure at a depth of 10,000 feet in the sea?

A. 760 mmHg
B. 523 mmHg (Correct Answer)
C. 349 mmHg
D. 226 mmHg

Explanation: **Explanation:** The barometric (atmospheric) pressure decreases exponentially as altitude increases. This is a fundamental concept in high-altitude physiology. At sea level, the standard barometric pressure is **760 mmHg**. As one ascends, the weight of the air column above decreases, leading to a fall in pressure and a subsequent decrease in the partial pressure of oxygen ($PO_2$), which can lead to hypoxia. At an altitude of **10,000 feet**, the barometric pressure is approximately **523 mmHg**. This is a high-yield value to remember for NEET-PG, as it represents the threshold where compensatory physiological mechanisms (like hyperventilation) become significant. **Analysis of Options:** * **Option A (760 mmHg):** This is the standard pressure at **sea level**. * **Option B (523 mmHg):** **Correct.** This is the pressure at 10,000 feet. * **Option C (349 mmHg):** This is the approximate pressure at **20,000 feet** (e.g., near the summit of Mt. Kilimanjaro). * **Option D (226 mmHg):** This is the approximate pressure at **30,000 feet** (near the summit of Mt. Everest). **High-Yield Clinical Pearls for NEET-PG:** 1. **Fraction of Oxygen ($FiO_2$):** Remains constant at **21%** regardless of altitude; only the total barometric pressure (and thus $PO_2$) changes. 2. **Alveolar Gas Equation:** As barometric pressure falls, $PAO_2$ falls. At 10,000 feet, $PAO_2$ drops to about 60-67 mmHg (compared to 100 mmHg at sea level). 3. **Acute Mountain Sickness (AMS):** Usually begins to manifest at altitudes above 8,000 feet. 4. **Rule of Thumb:** Barometric pressure roughly halves for every 18,000 feet of ascent.

Question 277: Which of the following is a characteristic of irreversible cell injury?

A. Mitochondrial densities (Correct Answer)
B. Cellular swelling
C. Blebs
D. None of the above

Explanation: ### Explanation The transition from reversible to irreversible cell injury is marked by two critical phenomena: the inability to reverse mitochondrial dysfunction and profound disturbances in membrane function. **1. Why "Mitochondrial Densities" is correct:** The presence of **large, flocculent, amorphous densities** in the mitochondrial matrix is a hallmark of **irreversible cell injury**. These densities represent the precipitation of proteins and the deposition of calcium phosphate. While mitochondrial *swelling* can occur in reversible injury, the formation of these permanent, opaque densities indicates that the cell has passed the "point of no return" and is progressing toward necrosis. **2. Why the other options are incorrect:** * **Cellular Swelling (Hydropic Change):** This is the **earliest manifestation** of almost all forms of injury to cells. It results from the failure of energy-dependent ion pumps (like the Na+/K+ ATPase) in the plasma membrane, leading to an influx of water. It is a hallmark of **reversible injury**. * **Blebs:** Plasma membrane alterations, such as blebbing, blunting, and loss of microvilli, are characteristic features of **reversible injury**. While membrane damage becomes more severe in irreversible injury (leading to rupture), simple blebbing is still considered reversible if the stressor is removed. **High-Yield Clinical Pearls for NEET-PG:** * **Point of No Return:** Irreversible injury is characterized by **severe mitochondrial damage** and **lysosomal membrane rupture** (leading to enzymatic digestion of the cell). * **Nuclear Changes:** Irreversible injury involves definitive nuclear changes: **Pyknosis** (shrinkage/condensation), **Karyorrhexis** (fragmentation), and **Karyolysis** (dissolution). * **Myocardial Infarction:** In cardiac myocytes, irreversible injury (necrosis) typically occurs after **20–40 minutes** of severe ischemia. * **Morphological Signpost:** If you see "Amorphous densities" or "Flocculent densities" in a question stem regarding electron microscopy, think **Irreversible Injury/Necrosis**.

Question 278: What provides the cell shape and physical structure?

A. Cell membrane
B. Microtubules (Correct Answer)
C. Microfilaments
D. Golgi apparatus

Explanation: ### Explanation The cytoskeleton is a complex network of protein filaments that maintains the structural integrity of the cell. Among its components, **Microtubules** are the primary determinants of cell shape and physical structure. **1. Why Microtubules are correct:** Microtubules are hollow, rigid cylinders made of alpha and beta-tubulin dimers. They act as the "scaffolding" or internal skeleton of the cell. Their high rigidity allows them to resist compression forces, thereby maintaining the cell's three-dimensional shape. They also facilitate intracellular transport (via kinesin and dynein) and form the core of cilia and flagella. **2. Why other options are incorrect:** * **Cell membrane:** While it defines the boundary of the cell and regulates transport, it is a fluid phospholipid bilayer. It lacks the structural rigidity to maintain shape on its own; it relies on the underlying cytoskeleton for support. * **Microfilaments (Actin):** These are primarily involved in cell movement, muscle contraction, and cytokinesis. While they provide mechanical support to the cell surface (forming the terminal web), they are not the primary structural "struts" like microtubules. * **Golgi apparatus:** This is a membrane-bound organelle involved in the processing, packaging, and trafficking of proteins. It has no role in providing mechanical structure to the cell. **Clinical Pearls for NEET-PG:** * **Drugs acting on Microtubules:** Remember the mnemonic **"Microtubules Get Vine-like Cold"** for drugs that inhibit polymerization/depolymerization: **M**ebendazole, **G**riseofulvin, **V**incristine/Vinblastine, **C**olchicine, and **T**axanes (Paclitaxel). * **Kartagener Syndrome:** Caused by a defect in dynein arms within microtubules, leading to immobile cilia, situs inversus, and bronchiectasis. * **Intermediate Filaments:** These provide tensile strength (e.g., Keratin in epithelial cells, Desmin in muscle).

Question 279: What is a serious complication of a prolonged sitting posture?

A. Dysrhythmia
B. Hypotension
C. Nerve paralysis
D. Venous embolism (Correct Answer)

Explanation: **Explanation:** The correct answer is **Venous embolism**. This occurs due to the physiological consequences of prolonged immobilization, often referred to as "Economy Class Syndrome." **1. Why Venous Embolism is correct:** Prolonged sitting leads to **venous stasis** in the lower limbs. According to **Virchow’s Triad** (Stasis, Hypercoagulability, and Endothelial injury), reduced blood flow is a primary trigger for thrombus formation. In a sitting posture, the "peripheral heart" (the calf muscle pump) is inactive, and the popliteal vein may be mechanically compressed. This leads to **Deep Vein Thrombosis (DVT)**. If a portion of this thrombus dislodges, it becomes an embolus, potentially causing a life-threatening **Pulmonary Embolism (PE)**. **2. Why other options are incorrect:** * **Dysrhythmia:** While electrolyte imbalances can cause arrhythmias, prolonged sitting does not directly trigger cardiac rhythm disturbances. * **Hypotension:** Prolonged *standing* is more commonly associated with orthostatic hypotension or vasovagal syncope. Sitting usually maintains adequate venous return to prevent acute hypotension. * **Nerve paralysis:** While localized nerve compression (e.g., peroneal nerve palsy) can occur from crossing legs, it is generally considered a localized injury rather than a "serious systemic complication" compared to the mortality risk of an embolism. **High-Yield Clinical Pearls for NEET-PG:** * **Virchow’s Triad:** Essential for understanding DVT/PE pathogenesis. * **Calf Muscle Pump:** The gastrocnemius and soleus muscles are vital for venous return against gravity. * **Homan’s Sign:** Pain in the calf on dorsiflexion of the foot (classic but non-specific sign of DVT). * **Gold Standard Investigation:** Duplex Ultrasonography for DVT; CT Pulmonary Angiography (CTPA) for Pulmonary Embolism.

Question 280: Which of the following is true regarding transport across a cell membrane?

A. Chloride with glucose symport
B. Sodium with glucose antiport
C. Sodium with glucose symport (Correct Answer)
D. Potassium with glucose symport

Explanation: **Explanation:** The correct answer is **Sodium with glucose symport (Option C)**. This process is a classic example of **Secondary Active Transport**, specifically **Cotransport (Symport)**. In this mechanism, glucose is transported against its concentration gradient by "hitching a ride" with sodium ions. Sodium moves down its electrochemical gradient (created by the Na⁺-K⁺ ATPase pump), providing the energy required to pull glucose into the cell. This occurs via specific carrier proteins known as **SGLT (Sodium-Glucose Linked Transporters)**. **Analysis of Options:** * **Option A & D:** Chloride and Potassium are not the primary ions coupled with glucose transport in human physiology. Sodium is the unique driving ion because of the steep extracellular-to-intracellular gradient maintained by the cell. * **Option B:** An **Antiport** (Counter-transport) moves substances in opposite directions (e.g., Na⁺-H⁺ exchanger). Glucose and Sodium move in the *same* direction (into the cell), making it a **Symport**. **High-Yield Clinical Pearls for NEET-PG:** 1. **SGLT-1:** Located in the **Small Intestine** (enterocytes) and the late proximal tubule of the kidney. It is responsible for glucose absorption from the gut. 2. **SGLT-2:** Located in the **Early Proximal Convoluted Tubule (PCT)** of the kidney. It reabsorbs ~90% of filtered glucose. 3. **Clinical Correlation:** **SGLT-2 Inhibitors** (e.g., Dapagliflozin) are used in Type 2 Diabetes to induce glucosuria and lower blood sugar. 4. **Oral Rehydration Therapy (ORT):** The efficacy of ORS is based on the SGLT-1 receptor; sodium and glucose are given together to maximize water absorption via osmosis.

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