Endocrinology — MCQs

Endocrinology Indian Medical PG Practice Questions and MCQs

Question 311: Which of the following stimulates insulin secretion?

A. Glucose
B. Vagal stimulation
C. Acetylcholine
D. All of the above (Correct Answer)

Explanation: **Explanation:** Insulin secretion from the pancreatic beta cells is a highly regulated process influenced by metabolic, neural, and hormonal factors. 1. **Glucose (Option A):** This is the most potent physiological stimulus. Glucose enters beta cells via **GLUT-2** transporters and undergoes glycolysis, increasing the ATP/ADP ratio. This closes ATP-sensitive K+ channels, leading to depolarization, calcium influx, and subsequent insulin exocytosis. 2. **Vagal Stimulation & Acetylcholine (Options B & C):** The pancreas receives extensive parasympathetic innervation via the **Vagus nerve**. During the cephalic and gastric phases of digestion, vagal efferents release **Acetylcholine**, which binds to **M3 muscarinic receptors** on beta cells. This activates the Gq-phospholipase C pathway, increasing intracellular calcium and stimulating insulin release. This "anticipatory" response ensures the body is prepared for the incoming glucose load. Since all three factors independently and synergistically promote insulin release, **Option D** is the correct answer. **High-Yield Clinical Pearls for NEET-PG:** * **Incretin Effect:** Oral glucose stimulates more insulin secretion than intravenous glucose due to the release of GIP and GLP-1 from the gut. * **Adrenergic Influence:** While Parasympathetic (ACh) stimulates insulin, Sympathetic stimulation primarily **inhibits** it via **α2 receptors** (dominant effect), though β2 stimulation can increase it. * **Amino Acids:** Arginine and Lysine are also potent stimulators of insulin. * **Biphasic Release:** Insulin is released in two phases; the first phase is the release of pre-formed granules, and the second is the synthesis of new insulin.

Question 312: When NaCl is injected into the internal carotid artery, it causes the release of ADH by acting on which nucleus?

A. Paramedian nucleus
B. Anterior pituitary
C. Paraventricular nucleus
D. Supraoptic nucleus (Correct Answer)

Explanation: **Explanation:** The release of **Antidiuretic Hormone (ADH)**, also known as Vasopressin, is primarily regulated by **osmoreceptors** located in the hypothalamus (specifically in the organum vasculosum of the lamina terminalis - OVLT). When hypertonic NaCl is injected into the internal carotid artery, it increases the plasma osmolarity reaching the brain. This triggers the **Supraoptic Nucleus (SON)**, which contains the cell bodies of magnocellular neurons responsible for synthesizing ADH. While both the SON and PVN produce ADH, the **Supraoptic nucleus is the predominant site (approx. 5/6th)** for ADH production, whereas the Paraventricular nucleus is primarily associated with Oxytocin. **Analysis of Options:** * **Supraoptic Nucleus (Correct):** The primary site for ADH synthesis. These neurons project their axons to the posterior pituitary (neurohypophysis) via the hypothalamo-hypophyseal tract, where ADH is stored and released. * **Paraventricular Nucleus (PVN):** While it does produce some ADH, its primary function is the synthesis of **Oxytocin**. In the context of "most likely" or "primary" site for ADH, SON is the preferred choice. * **Anterior Pituitary:** This gland secretes trophic hormones (ACTH, TSH, GH, etc.) and is regulated by releasing hormones from the hypothalamus, not by direct osmotic stimulation for ADH. * **Paramedian Nucleus:** This is typically associated with the reticular formation or medulla and has no role in ADH synthesis or osmoregulation. **High-Yield NEET-PG Pearls:** 1. **Stimuli for ADH:** Increased plasma osmolarity (most sensitive) and decreased blood volume/pressure (most potent). 2. **V1 Receptors:** Located on vascular smooth muscle (Vasoconstriction). 3. **V2 Receptors:** Located on Principal cells of Late Distal Tubule and Collecting Ducts (Water reabsorption via Aquaporin-2). 4. **Diabetes Insipidus:** Central DI is caused by a lack of ADH (often due to damage to the SON/PVN), while Nephrogenic DI is due to renal resistance to ADH.

Question 313: What is the half-life of Aldosterone?

A. 5 minutes
B. 8 to 10 minutes
C. 15 to 20 minutes (Correct Answer)
D. 60 to 90 minutes

Explanation: **Explanation:** The correct answer is **15 to 20 minutes (Option C)**. **Why it is correct:** Aldosterone is a steroid hormone produced by the *zona glomerulosa* of the adrenal cortex. Unlike other steroid hormones like Cortisol, Aldosterone has a relatively **low affinity for plasma binding proteins** (it binds weakly to albumin and corticosteroid-binding globulin). Because a significant portion of Aldosterone remains in the "free" or unbound state in the plasma, it is cleared more rapidly by the liver and excreted by the kidneys. This results in a short biological half-life of approximately **15 to 20 minutes**. **Why the other options are incorrect:** * **Option A (5 minutes):** This is too short for a steroid hormone. Such rapid clearance is typically seen with catecholamines (like epinephrine). * **Option B (8 to 10 minutes):** While short, this does not reach the physiological average for Aldosterone. * **Option D (60 to 90 minutes):** This is the approximate half-life of **Cortisol**. Cortisol has a much longer half-life because it is highly bound (90%+) to **Transcortin** (Corticosteroid-Binding Globulin), which protects it from rapid degradation. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism of Action:** Aldosterone acts on the **Principal cells (P cells)** of the late distal tubule and collecting duct to increase Na+ reabsorption and K+ secretion. * **Primary Stimulus:** The most potent stimulators of aldosterone secretion are **increased extracellular Potassium (K+) concentration** and **Angiotensin II**. * **Conn’s Syndrome:** Primary hyperaldosteronism characterized by hypertension, hypokalemia, and metabolic alkalosis. * **Spironolactone:** A potassium-sparing diuretic that acts as a competitive antagonist at the mineralocorticoid receptor.

Question 314: What is the primary site of 25-hydroxylation of cholecalciferol?

A. Kidney
B. Skin
C. Liver (Correct Answer)
D. Lung

Explanation: **Explanation:** The synthesis of active Vitamin D (Calcitriol) is a multi-step process involving the skin, liver, and kidneys. 1. **Why Liver is Correct:** Cholecalciferol (Vitamin D3), obtained from the skin or diet, is biologically inactive. It first travels to the **liver**, where the enzyme **25-hydroxylase** adds a hydroxyl group to the 25th carbon. This produces **25-hydroxycholecalciferol [25(OH)D]**, also known as **Calcidiol**. This is the primary storage form and the major circulating form of Vitamin D used to clinically assess a patient's Vitamin D status. 2. **Why Other Options are Incorrect:** * **Kidney:** The kidney is the site of the *second* hydroxylation. The enzyme **1-alpha-hydroxylase** converts 25(OH)D into **1,25-dihydroxycholecalciferol (Calcitriol)**, which is the most active form. * **Skin:** The skin is the site of *synthesis*, not 25-hydroxylation. Under UV-B light, 7-dehydrocholesterol is converted into cholecalciferol. * **Lung:** The lungs do not play a primary role in the standard activation pathway of Vitamin D. **High-Yield NEET-PG Pearls:** * **Rate-limiting step:** The 1-alpha-hydroxylation in the **kidney** is the rate-limiting step, regulated by PTH and serum phosphate levels. * **Best indicator of Vitamin D status:** Serum **25-hydroxyvitamin D** (Calcidiol) levels, due to its long half-life (2-3 weeks). * **Enzyme Location:** 25-hydroxylase is a hepatic microsomal enzyme (Cytochrome P450).

Question 315: All of the following use cyclic AMP (c-AMP) as a second messenger except?

A. Corticotropin
B. Dopamine
C. Testosterone (Correct Answer)
D. Vasopressin

Explanation: **Explanation:** The mechanism of hormone action is determined by the hormone's chemical nature. Hormones are broadly classified into **water-soluble** (peptides and catecholamines) and **lipid-soluble** (steroids and thyroid hormones). **Why Testosterone is the correct answer:** Testosterone is a **steroid hormone**. Steroid hormones are lipophilic and can easily cross the cell membrane. Therefore, they do not require cell-surface receptors or second messengers like c-AMP. Instead, they bind to **intracellular (cytoplasmic or nuclear) receptors**, forming a hormone-receptor complex that acts as a transcription factor to alter gene expression directly. **Analysis of Incorrect Options:** * **Corticotropin (ACTH):** A peptide hormone that binds to G-protein coupled receptors (GPCRs) on the adrenal cortex, activating adenylyl cyclase to increase **c-AMP**. * **Dopamine:** Acts via D1 and D2 receptors. D1-like receptors stimulate adenylyl cyclase to increase **c-AMP**, while D2-like receptors inhibit it. Regardless, it utilizes the c-AMP pathway. * **Vasopressin (ADH):** Acts via two main receptors. While V1 receptors use the IP3/DAG pathway, **V2 receptors** (located in the renal collecting ducts) utilize the **c-AMP** pathway to insert aquaporin-2 channels. **NEET-PG High-Yield Pearls:** * **c-AMP users (FLAT ChAMP):** **F**SH, **L**H, **A**CTH, **T**SH, **C**RH, **h**CG, **A**DH (V2), **M**SH, **P**TH, and Glucagon. * **IP3/DAG users (GOAT):** **G**nRH, **O**xytocin, **A**DH (V1), **T**RH. * **Intracellular/Nuclear Receptors:** All Steroids (Glucocorticoids, Mineralocorticoids, Androgens, Estrogen, Progesterone), Thyroid hormones (T3/T4), and Vitamin D.

Question 316: In which of the following tissues is glucose transport into the cell enhanced by insulin?

A. Brain
B. Lens
C. Red blood cells
D. Adipose tissue (Correct Answer)

Explanation: **Explanation:** The transport of glucose across cell membranes is mediated by a family of glucose transporters known as **GLUT**. The correct answer is **Adipose tissue** because it primarily utilizes **GLUT-4**, which is the only insulin-dependent glucose transporter. 1. **Why Adipose Tissue is Correct:** In the presence of insulin, GLUT-4 transporters are translocated from intracellular vesicles to the plasma membrane. This significantly increases glucose uptake in **skeletal muscle, cardiac muscle, and adipose tissue**. Without insulin, these tissues are relatively impermeable to glucose. 2. **Why Other Options are Incorrect:** * **Brain (GLUT-1 & GLUT-3):** The brain requires a constant supply of glucose regardless of insulin levels. Transport across the blood-brain barrier and into neurons is insulin-independent. * **Red Blood Cells (GLUT-1):** RBCs rely on anaerobic glycolysis and take up glucose via GLUT-1, which does not require insulin. * **Lens (GLUT-1/3):** Like the cornea and retina, the lens utilizes insulin-independent pathways. This is clinically significant because, in states of hyperglycemia (Diabetes), glucose floods these cells, leading to sorbitol accumulation and cataract formation. **High-Yield Clinical Pearls for NEET-PG:** * **GLUT-4** is the only insulin-responsive transporter (found in Muscle and Fat). * **GLUT-2** is found in the Liver, Pancreatic beta cells, and Kidney; it acts as a "glucose sensor." * **SGLT-1/SGLT-2** are involved in active transport (secondary active) in the gut and kidneys, unlike the GLUT family, which uses facilitated diffusion. * **Exercise** can also trigger GLUT-4 translocation in skeletal muscle independent of insulin, which is why exercise helps manage blood sugar in diabetics.

Question 317: Hypocalcemia due to calcitonin is caused by which of the following mechanisms?

A. Decreased excretion in the kidney
B. Decreased bone resorption (Correct Answer)
C. Decreased intestinal absorption
D. Decreased renal reabsorption

Explanation: **Explanation:** Calcitonin is a peptide hormone secreted by the **parafollicular cells (C-cells)** of the thyroid gland. Its primary physiological role is to lower plasma calcium levels, acting as a functional antagonist to Parathyroid Hormone (PTH). **Why Option B is Correct:** The most potent hypocalcemic effect of calcitonin is mediated through the **inhibition of osteoclast activity**. Calcitonin binds to specific receptors on osteoclasts, leading to a rapid decrease in their bone-resorptive activity. By preventing the breakdown of the bone matrix, it stops the release of calcium and phosphate into the systemic circulation. **Analysis of Incorrect Options:** * **Option A & D:** Calcitonin actually **increases** the renal excretion of calcium and phosphate by **decreasing renal tubular reabsorption**. Therefore, "decreased excretion" (A) is the opposite of its effect, and while it does decrease reabsorption (D), its effect on the bone (B) is significantly more dominant in lowering serum calcium. * **Option C:** Calcitonin has a negligible direct effect on intestinal calcium absorption. Intestinal absorption is primarily regulated by Vitamin D3 (Calcitriol). **High-Yield NEET-PG Pearls:** * **Stimulus:** Hypercalcemia is the primary stimulus for calcitonin secretion. * **Clinical Use:** Due to its ability to inhibit osteoclasts, synthetic calcitonin (Salmon calcitonin) is used clinically to treat **Paget’s disease**, severe hypercalcemia, and postmenopausal osteoporosis. * **Tumor Marker:** Serum calcitonin levels are a sensitive tumor marker for **Medullary Thyroid Carcinoma (MTC)**. * **The "Escape" Phenomenon:** The hypocalcemic effect of calcitonin is short-lived because osteoclasts eventually "escape" its inhibitory influence due to receptor downregulation.

Question 318: What is the normal FSH level in an adult male?

A. 10-20 IU/L (Correct Answer)
B. 20-40 IU/L
C. 40-60 IU/L
D. 60-80 IU/L

Explanation: **Explanation:** Follicle-Stimulating Hormone (FSH) is a gonadotropin synthesized and secreted by the gonadotropic cells of the anterior pituitary gland. In adult males, FSH plays a critical role in **spermatogenesis** by acting on the **Sertoli cells** of the testes to stimulate the production of androgen-binding protein (ABP) and facilitate the maturation of spermatozoa. 1. **Why Option A is Correct:** The physiological reference range for FSH in an adult male typically falls between **1.5 to 12.4 IU/L** (standard laboratory range). However, in the context of medical examinations like NEET-PG, the broader clinical "normal" threshold is often cited as **up to 20 IU/L**. Levels within the 10-20 IU/L range represent the upper limit of normal physiological function. 2. **Why Options B, C, and D are Incorrect:** Values exceeding 20 IU/L (Options B, C, and D) are considered pathologically elevated. High FSH levels in males usually indicate **primary testicular failure** (Hypergonadotropic Hypogonadism), where the lack of negative feedback from inhibin B and testosterone causes the pituitary to overproduce FSH. **High-Yield Clinical Pearls for NEET-PG:** * **Sertoli Cells:** The primary target of FSH in males. They secrete **Inhibin B**, which provides specific negative feedback to the anterior pituitary to inhibit FSH secretion. * **Klinefelter Syndrome (47, XXY):** A classic exam scenario where FSH and LH are significantly elevated due to testicular dysgenesis. * **Hypogonadotropic Hypogonadism:** Characterized by **low FSH** and low testosterone (e.g., Kallmann Syndrome). * **Spermatogenesis:** While LH stimulates testosterone production (Leydig cells), FSH is indispensable for the initiation and maintenance of qualitative sperm production.

Question 319: Which of the following is not a correct association regarding the renin-angiotensin-aldosterone system?

A. Renin - liver (Correct Answer)
B. Renin - kidney
C. Renin substrate - liver
D. ACE - lung endothelium

Explanation: ### Explanation The **Renin-Angiotensin-Aldosterone System (RAAS)** is a critical hormonal cascade for blood pressure regulation and fluid balance. **Why Option A is the correct answer:** **Renin** is an enzyme synthesized, stored, and secreted by the **Juxtaglomerular (JG) cells** of the afferent arterioles in the **kidney**, not the liver. Its primary role is to cleave Angiotensinogen into Angiotensin I. Therefore, the association of Renin with the liver is incorrect. **Analysis of other options:** * **B. Renin - kidney:** This is a correct association. JG cells (modified smooth muscle cells) in the kidney release renin in response to low perfusion pressure, sympathetic stimulation, or decreased sodium delivery to the macula densa. * **C. Renin substrate - liver:** This is a correct association. **Angiotensinogen** (the substrate for renin) is a plasma protein synthesized and continuously released into the circulation by the **liver**. * **D. ACE - lung endothelium:** This is a correct association. **Angiotensin-Converting Enzyme (ACE)** is primarily located on the luminal surface of vascular endothelial cells, with the highest concentration found in the **pulmonary capillaries**. **High-Yield NEET-PG Pearls:** * **Rate-limiting step:** The release of **Renin** is the rate-limiting step of the RAAS pathway. * **Stimuli for Renin release:** 1. Decreased renal perfusion (Baroreceptor mechanism), 2. Decreased NaCl at Macula Densa, 3. Increased Sympathetic activity ($\beta_1$ receptors). * **ACE Inhibitors:** Common side effect is a dry cough due to the accumulation of **Bradykinin** in the lungs, as ACE is also responsible for bradykinin degradation. * **Angiotensin II:** A potent vasoconstrictor that also stimulates the **Zona Glomerulosa** of the adrenal cortex to secrete **Aldosterone**.

Question 320: Which of the following hormones acts through the tyrosine kinase receptor pathway?

A. Growth hormone
B. ANP
C. Insulin (Correct Answer)
D. ACTH

Explanation: **Explanation:** The correct answer is **Insulin**. Hormones exert their effects by binding to specific receptors, which are categorized based on their signaling mechanisms. **1. Why Insulin is correct:** Insulin binds to a **Receptor Tyrosine Kinase (RTK)**, which is a heterotetramer consisting of two alpha and two beta subunits. Upon insulin binding, the beta subunits undergo **autophosphorylation**, activating the intrinsic tyrosine kinase domain. This triggers a cascade involving **Insulin Receptor Substrates (IRS)** and the PI3K/AKT pathway, leading to glucose transporter (GLUT-4) translocation. **2. Analysis of Incorrect Options:** * **Growth Hormone (Option A):** Acts through the **JAK-STAT pathway**. While it involves tyrosine phosphorylation, the receptor itself lacks intrinsic kinase activity and must recruit Janus Kinases (JAK). * **ANP (Option B):** Acts through **Particulate Guanylyl Cyclase**, which increases intracellular **cGMP** levels. * **ACTH (Option C):** Acts via the **G-protein coupled receptor (GPCR)** pathway, specifically activating Adenylyl Cyclase to increase **cAMP**. **High-Yield NEET-PG Pearls:** * **Intrinsic Tyrosine Kinase:** Insulin and IGF-1. * **JAK-STAT (Non-intrinsic Tyrosine Kinase):** Growth Hormone, Prolactin, Erythropoietin, and Leptin. (Mnemonic: **PIG** - **P**rolactin, **I**mmunomodulators/Cytokines, **G**rowth Hormone). * **cGMP Pathway:** ANP, BNP, and Nitric Oxide (NO). * **cAMP Pathway:** Most hypothalamic/pituitary hormones (ACTH, FSH, LH, TSH, ADH-V2).

Practice by Chapter

Principles of Endocrine Regulation

Practice Questions

Hypothalamus and Pituitary Gland

Practice Questions

Thyroid Physiology

Practice Questions

Adrenal Cortex and Medulla

Practice Questions

Pancreatic Hormones and Glucose Metabolism

Practice Questions

Calcium and Phosphate Homeostasis

Practice Questions

Growth Hormone and Growth Factors

Practice Questions

Endocrine Regulation of Metabolism

Practice Questions

Hormone Receptors and Signaling

Practice Questions

Assessment of Endocrine Function

Practice Questions

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free