Endocrinology — MCQs

Endocrinology Indian Medical PG Practice Questions and MCQs

Question 251: Heat acclimatization is primarily due to which hormone?

A. Thyroxine
B. Insulin
C. Adrenaline
D. Aldosterone (Correct Answer)

Explanation: **Explanation:** Heat acclimatization is the process by which the body undergoes physiological adaptations to improve heat tolerance. The primary hormonal driver for this process is **Aldosterone**. **Why Aldosterone is Correct:** When an individual is repeatedly exposed to high temperatures, the body aims to minimize electrolyte loss while maximizing cooling through sweat. Aldosterone acts on the epithelial cells of the sweat glands to increase the **reabsorption of Sodium (Na+) and Chloride (Cl-)**. This results in: 1. **Conservation of salt:** Preventing hyponatremia during profuse sweating. 2. **Increased sweat volume:** By preserving total body salt, the body can maintain plasma volume, allowing for a higher sweat rate and better evaporative cooling. **Analysis of Incorrect Options:** * **Thyroxine (A):** This hormone increases the Basal Metabolic Rate (BMR) and heat production (thermogenesis). In hot environments, thyroid activity actually tends to decrease to prevent further heat load. * **Insulin (B):** Primarily regulates glucose metabolism and has no direct role in thermoregulation or electrolyte conservation during heat stress. * **Adrenaline (C):** While it can stimulate sweating (diaphoresis) during acute stress or "fight or flight" responses, it is not responsible for the long-term physiological adaptation seen in acclimatization. **Clinical Pearls for NEET-PG:** * **Acclimatization Timeline:** It typically takes **7 to 14 days** of exposure to a hot environment. * **Sweat Composition:** An acclimatized person produces sweat that is more **hypotonic** (lower salt concentration) compared to a non-acclimatized person. * **Plasma Volume:** Acclimatization leads to an expansion of plasma volume (by 10-12%), which helps maintain cardiac output during exercise in the heat.

Question 252: Decreased protein to lipid ratio is seen in which of the following membranes?

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

Explanation: The protein-to-lipid ratio of a biological membrane is directly proportional to its metabolic activity. Membranes involved in active transport, signal transduction, or energy production have high protein content, while membranes acting primarily as insulators have high lipid content. **1. Why Myelin Sheath is Correct:** The myelin sheath serves as an electrical insulator for axons to facilitate saltatory conduction. To minimize ion leakage and provide maximum insulation, it is composed of approximately **80% lipids** (mainly cholesterol and sphingomyelin) and only **20% proteins**. This results in the **lowest protein-to-lipid ratio (0.25:1)** in the human body. **2. Why Other Options are Incorrect:** * **Inner Mitochondrial Membrane (IMM):** This membrane has the **highest protein-to-lipid ratio (3:1)**. It is packed with electron transport chain complexes and ATP synthase, reflecting its intense metabolic role in oxidative phosphorylation. * **Outer Mitochondrial Membrane (OMM):** While less protein-dense than the IMM, it still contains numerous porins and enzymes, maintaining a ratio of approximately **1:1**. * **Sarcoplasmic Reticulum:** This membrane is highly specialized for calcium signaling and contains a high density of SERCA pumps (proteins), resulting in a high protein-to-lipid ratio (approx. **2:1**). **High-Yield Clinical Pearls for NEET-PG:** * **Highest Protein Content:** Inner Mitochondrial Membrane (approx. 75-80% protein). * **Highest Lipid Content:** Myelin Sheath (approx. 80% lipid). * **Plasma Membrane Average:** Typically a 1:1 ratio (50% protein, 50% lipid). * **Key Lipid in Myelin:** Galactocerebroside is the characteristic glycolipid of myelin. * **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 253: Which of the following is NOT an effect of the T3 hormone?

A. It increases the heart rate
B. It increases the stroke volume
C. It decreases the peripheral resistance
D. It decreases pulse pressure (Correct Answer)

Explanation: **Explanation:** The thyroid hormone (T3) has profound effects on the cardiovascular system, primarily acting to increase cardiac output and meet the body's metabolic demands. **Why Option D is the Correct Answer:** T3 **increases pulse pressure**, it does not decrease it. Pulse pressure is the difference between systolic blood pressure (SBP) and diastolic blood pressure (DBP). T3 increases SBP (due to increased stroke volume and contractility) and decreases DBP (due to vasodilation). This widening of the gap results in a **wide pulse pressure**, a classic clinical sign of hyperthyroidism. **Analysis of Incorrect Options:** * **A. Increases Heart Rate:** T3 has a direct chronotropic effect by upregulating $\beta_1$-adrenergic receptors in the heart and increasing the rate of depolarization in the SA node. * **B. Increases Stroke Volume:** T3 exerts a positive inotropic effect by increasing the expression of $\alpha$-myosin heavy chains and SERCA (sarcoplasmic reticulum calcium ATPase), enhancing myocardial contractility and stroke volume. * **C. Decreases Peripheral Resistance:** T3 causes direct vasodilation of peripheral arterioles by increasing local metabolic end-products and decreasing systemic vascular resistance (SVR). **NEET-PG High-Yield Pearls:** * **Hyperthyroidism:** Characterized by tachycardia, palpitations, and a "bounding pulse" (due to wide pulse pressure). * **Hypothyroidism:** Characterized by bradycardia and a "narrow pulse pressure." * **Molecular Mechanism:** T3 increases the transcription of the **SERCA2 pump**, which improves calcium sequestration during diastole, leading to faster relaxation (lusitropic effect) and stronger subsequent contraction.

Question 254: In tetany, hyperexcitability is due to which of the following mechanisms?

A. Low calcium levels cause increased sodium permeability. (Correct Answer)
B. Prevent potassium release.
C. Prevent sodium and intracellular calcium release.
D. Increased intracellular calcium produces the generation of an action potential.

Explanation: **Mechanism of Hypocalcemic Tetany** The correct answer is **A: Low calcium levels cause increased sodium permeability.** **Explanation:** In the extracellular fluid (ECF), calcium ions ($Ca^{2+}$) play a critical role in stabilizing the resting membrane potential of excitable tissues (nerve and muscle). Calcium ions normally bind to the external surface of voltage-gated sodium channels, increasing the voltage threshold required to open them. When ECF calcium levels drop (**hypocalcemia**), there are fewer positive charges to "mask" the negative charges on the cell membrane. This reduces the threshold for excitation, making the sodium channels much easier to open. Consequently, sodium permeability increases even with slight stimuli, leading to spontaneous depolarization and repetitive firing of action potentials. This manifests clinically as involuntary muscle contractions (tetany). **Analysis of Incorrect Options:** * **B & C:** These options are incorrect because tetany is primarily a phenomenon of the **cell membrane threshold** and sodium influx, rather than a direct inhibition of potassium release or intracellular calcium sequestration. * **D:** While intracellular calcium is required for muscle contraction (excitation-contraction coupling), the *hyperexcitability* (the tendency to fire) is triggered by the **extracellular** calcium deficit affecting sodium channel gating. **High-Yield NEET-PG Pearls:** * **Trousseau’s Sign:** Carpopedal spasm induced by inflating a BP cuff above systolic pressure (more sensitive than Chvostek's). * **Chvostek’s Sign:** Twitching of facial muscles upon tapping the facial nerve. * **ECG Finding:** Hypocalcemia characteristically causes **prolonged QT interval**. * **Rule of Thumb:** "Hypocalcemia excites, Hypercalcemia excites (the heart) but depresses (the nerves)."

Question 255: Which of the following is an autocrine hormone?

A. Prostaglandins
B. Thromboxane A2
C. Both Prostaglandins and Thromboxane A2 (Correct Answer)
D. None of the above

Explanation: ### Explanation The correct answer is **C: Both Prostaglandins and Thromboxane A2**. **Understanding the Concept:** Hormonal signaling is classified based on the distance and pathway the messenger travels to reach its target cell. * **Autocrine signaling** occurs when a cell secretes a hormone or chemical messenger that binds to receptors on the **same cell**, leading to changes within that cell. * **Paracrine signaling** involves messengers acting on **neighboring cells** within the local interstitial fluid. **Prostaglandins (PGs)** and **Thromboxane A2 (TXA2)** are derivatives of arachidonic acid (eicosanoids). They are classic examples of local hormones that exhibit both autocrine and paracrine activities. For instance, in platelet aggregation, Thromboxane A2 released by a platelet acts back on the same platelet (autocrine) to accelerate degranulation and on adjacent platelets (paracrine) to promote a pro-thrombotic state. Similarly, prostaglandins act locally on the smooth muscle or immune cells that secreted them to modulate inflammation and vascular tone. **Analysis of Options:** * **Option A & B:** While both are autocrine, selecting only one would be incomplete. Both belong to the eicosanoid family and share this signaling characteristic. * **Option D:** Incorrect, as both substances mentioned are well-documented autocrine regulators. **NEET-PG High-Yield Pearls:** 1. **Eicosanoids:** Prostaglandins, Thromboxanes, and Leukotrienes are generally considered local hormones (autocrine/paracrine) because they have a very short half-life and are rapidly inactivated in the circulation (especially in the lungs). 2. **Intracrine Signaling:** A subtype of autocrine signaling where the hormone acts *inside* the cell without ever being secreted (e.g., Calcitriol, certain growth factors). 3. **Key Example:** Insulin-like Growth Factor-1 (IGF-1) acts as an endocrine hormone (from liver) but also as an autocrine/paracrine factor in bone tissue.

Question 256: Which hormone stimulates the synthesis of milk proteins?

A. Luteinizing hormone (LH)
B. Prolactin (PRL) (Correct Answer)
C. Thyroid-stimulating hormone (TSH)
D. Growth hormone (GH)

Explanation: **Explanation:** **Prolactin (PRL)** is the primary hormone responsible for the **synthesis of milk proteins** (such as casein and lactalbumin) and the initiation of lactation (lactogenesis). It is secreted by the lactotrophs of the anterior pituitary. During pregnancy, high levels of estrogen and progesterone stimulate breast development but inhibit the actual secretion of milk. Postpartum, the sudden drop in these steroid hormones allows Prolactin to stimulate the mammary alveolar cells to produce milk. **Analysis of Incorrect Options:** * **Luteinizing hormone (LH):** A gonadotropin that triggers ovulation in females and stimulates testosterone production by Leydig cells in males. It has no direct role in milk synthesis. * **Thyroid-stimulating hormone (TSH):** Regulates the production of thyroid hormones (T3 and T4) from the thyroid gland to control basal metabolic rate. * **Growth hormone (GH):** While structurally similar to Prolactin and possessing some lactogenic properties, its primary role is promoting linear growth and protein synthesis via IGF-1. **High-Yield NEET-PG Pearls:** * **Dopamine** (Prolactin-Inhibiting Hormone) is the primary physiological regulator that keeps Prolactin levels low in non-pregnant individuals. * **Oxytocin** is responsible for the **milk-ejection reflex** (let-down reflex) via contraction of myoepithelial cells, whereas Prolactin is for **milk production**. * **Suckling** is the strongest stimulus for both Prolactin and Oxytocin release. * **Hyperprolactinemia** often presents with galactorrhea and secondary amenorrhea (due to inhibition of GnRH).

Question 257: A diet deficient in calcium will result in what physiological change?

A. Stimulation of the thyroid gland
B. Increased ability to cross-link fibrin
C. Production of Calcitonin and a low blood calcium level
D. Production of parathyroid hormone and bone resorption (Correct Answer)

Explanation: ### Explanation The body maintains serum ionized calcium within a very narrow range (8.5–10.5 mg/dL) through a tight feedback loop involving the parathyroid glands, kidneys, and bones. **Why Option D is Correct:** When dietary calcium is deficient, serum calcium levels drop (**hypocalcemia**). This is sensed by **Calcium-Sensing Receptors (CaSR)** on the chief cells of the parathyroid glands. In response, **Parathyroid Hormone (PTH)** is secreted. PTH acts to restore calcium levels via three main mechanisms: 1. **Bone Resorption:** It stimulates osteoclasts (indirectly via RANKL) to break down bone matrix, releasing calcium into the blood. 2. **Renal Reabsorption:** It increases calcium reabsorption in the distal convoluted tubule. 3. **Vitamin D Activation:** It stimulates 1-alpha-hydroxylase in the kidney to produce **Calcitriol** (active Vitamin D), which increases intestinal calcium absorption. **Why Other Options are Incorrect:** * **Option A:** The thyroid gland (specifically follicular cells) regulates basal metabolic rate via T3/T4, not calcium. * **Option B:** Calcium (Factor IV) is essential for the coagulation cascade. A deficiency would **decrease** the ability to cross-link fibrin, potentially leading to bleeding tendencies, not increase it. * **Option C:** **Calcitonin** is secreted by the parafollicular (C-cells) of the thyroid in response to *hypercalcemia* (high calcium) to lower blood levels; it would be suppressed in a calcium-deficient state. **High-Yield NEET-PG Pearls:** * **PTH** is the single most important hormone for acute calcium regulation. * **Bone Resorption** is a "trade-off": the body prioritizes serum calcium (for nerve/muscle function) at the expense of bone density. * **Clinical Sign:** Hypocalcemia presents with **Chvostek’s sign** (facial twitching) and **Trousseau’s sign** (carpal spasm). * **Vitamin D** is essential for PTH to exert its effect on the intestines.

Question 258: What is the naturally occurring glucocorticoid?

A. Hydrocortisone
B. Cortisol (Correct Answer)
C. Prednisolone
D. Corticosterone

Explanation: **Explanation:** The adrenal cortex consists of three layers, with the **Zona Fasciculata** being responsible for the secretion of glucocorticoids. In humans, **Cortisol** is the primary, most potent, and most abundant naturally occurring glucocorticoid. It plays a vital role in glucose homeostasis (gluconeogenesis), protein catabolism, and modulating the inflammatory response. **Analysis of Options:** * **Cortisol (Option B):** This is the principal endogenous glucocorticoid in humans. It is synthesized from cholesterol and regulated by the Hypothalamic-Pituitary-Adrenal (HPA) axis via ACTH. * **Hydrocortisone (Option A):** While chemically identical to cortisol, the term "Hydrocortisone" is typically used in a **pharmacological context** to refer to the pharmaceutical preparation of the hormone used for replacement therapy or topical application. * **Prednisolone (Option C):** This is a **synthetic** glucocorticoid. It is a derivative of cortisol designed to have higher anti-inflammatory potency and less mineralocorticoid activity. * **Corticosterone (Option D):** While it is a naturally occurring glucocorticoid, it is the primary glucocorticoid in **rodents** (rats/mice). In humans, it serves mainly as an intermediate in the aldosterone synthesis pathway and has very weak glucocorticoid activity. **High-Yield NEET-PG Pearls:** 1. **Diurnal Variation:** Cortisol levels are highest in the early morning (approx. 8 AM) and lowest at midnight. 2. **Transport:** 90% of cortisol is bound to **CBG (Corticosteroid-Binding Globulin)** or Transcortin; only the free fraction is biologically active. 3. **Metabolism:** Cortisol is metabolized in the liver and excreted in the urine as **17-hydroxycorticosteroids**. 4. **Potency Ratio:** Prednisolone is roughly 4 times more potent than Cortisol, while Dexamethasone (synthetic) is about 25–30 times more potent.

Question 259: The cortisol level returns to normal after hemorrhage in how many days?

A. 2 weeks
B. 10 days
C. 7 days
D. 3 days (Correct Answer)

Explanation: **Explanation:** The correct answer is **3 days (Option D)**. **Underlying Medical Concept:** Hemorrhage acts as a potent physical stressor that triggers the **Hypothalamic-Pituitary-Adrenal (HPA) axis**. Following acute blood loss, there is an immediate and sharp increase in the secretion of **Corticotropin-Releasing Hormone (CRH)** and **Adrenocorticotropic Hormone (ACTH)**, leading to elevated plasma cortisol levels. Cortisol plays a vital role in the "fight or flight" response by maintaining vascular reactivity to catecholamines and mobilizing energy stores (gluconeogenesis) to cope with the stress of volume loss. In a typical physiological recovery following a non-fatal hemorrhage, the HPA axis stabilizes, and cortisol levels generally return to their baseline (pre-stress) values within **72 hours (3 days)**. **Analysis of Incorrect Options:** * **Option A (2 weeks) & Option B (10 days):** These timeframes are too long. While the body may take weeks to fully restore red blood cell mass (erythropoiesis), the hormonal/endocrine response to acute stress is much more rapid. * **Option C (7 days):** While some metabolic parameters take a week to normalize, the acute surge of cortisol specifically subsides much earlier as the immediate hemodynamic stress is managed. **NEET-PG High-Yield Pearls:** * **Cortisol & Catecholamines:** Cortisol has a "permissive effect" on catecholamines; it is essential for vasoconstriction. In its absence (Addisonian crisis), patients develop refractory hypotension. * **Diurnal Variation:** Remember that cortisol normally peaks at **8:00 AM** and is lowest at **midnight**. This rhythm is often lost during acute stress or in Cushing’s syndrome. * **Aldosterone vs. Cortisol:** While cortisol returns to normal in 3 days, aldosterone levels may remain elevated longer if the renin-angiotensin-aldosterone system (RAAS) is still compensating for residual volume deficit.

Question 260: Which of the following is NOT associated with progesterone?

A. Ovulation
B. Proliferative phase of the menstrual cycle (Correct Answer)
C. Thermogenesis
D. Secretory phase of the menstrual cycle

Explanation: **Explanation:** The correct answer is **B. Proliferative phase of the menstrual cycle.** The menstrual cycle is divided into two main phases: the **proliferative (follicular) phase** and the **secretory (luteal) phase**. The proliferative phase is dominated by **Estrogen**, which is secreted by the developing ovarian follicles to stimulate endometrial growth. Progesterone levels remain baseline during this phase. **Analysis of Options:** * **A. Ovulation:** While LH triggers ovulation, the process is preceded by a small pre-ovulatory rise in progesterone. Furthermore, the presence of progesterone is the definitive marker that ovulation has occurred, as it is secreted by the resulting Corpus Luteum. * **C. Thermogenesis:** Progesterone has a direct effect on the hypothalamus, increasing the basal body temperature (BBT) by approximately 0.5°F (0.3°C) after ovulation. This is a classic clinical marker used to track the luteal phase. * **D. Secretory phase:** This phase is entirely dependent on Progesterone. It transforms the estrogen-primed endometrium into a secretory state, making it receptive for implantation by increasing vascularity and glycogen storage. **High-Yield NEET-PG Pearls:** * **Source:** Progesterone is primarily secreted by the **Corpus Luteum** (stimulated by LH) and later by the **Placenta** (after 8–10 weeks). * **Pregnancy:** It is the "hormone of pregnancy" because it maintains endometrial integrity and decreases uterine contractility (quiescence). * **Diagnostic Test:** A serum progesterone level >3 ng/mL on Day 21 of the cycle is the gold standard for confirming that ovulation has occurred.

Practice by Chapter

Principles of Endocrine Regulation

Practice Questions

Hypothalamus and Pituitary Gland

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Thyroid Physiology

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Adrenal Cortex and Medulla

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Pancreatic Hormones and Glucose Metabolism

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Calcium and Phosphate Homeostasis

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Growth Hormone and Growth Factors

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Endocrine Regulation of Metabolism

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Hormone Receptors and Signaling

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Assessment of Endocrine Function

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