Endocrinology — MCQs

Endocrinology Indian Medical PG Practice Questions and MCQs

Question 221: What are important stimuli for prolactin secretion?

A. Insulin
B. Growth hormone
C. Dopamine
D. Estrogen (Correct Answer)

Explanation: **Explanation:** **1. Why Estrogen is Correct:** Estrogen is a potent stimulator of prolactin secretion. It acts directly on the anterior pituitary to stimulate the hypertrophy and hyperplasia of **lactotrophs** (prolactin-secreting cells). Furthermore, estrogen decreases the sensitivity of lactotrophs to the inhibitory effects of dopamine and directly stimulates the expression of the prolactin gene. This is clinically evident during pregnancy, where rising estrogen levels lead to a significant increase in prolactin levels to prepare the breasts for lactation. **2. Why the Other Options are Incorrect:** * **Dopamine (Option C):** This is the primary **inhibitor** of prolactin. Unlike other anterior pituitary hormones, prolactin is under tonic inhibitory control by dopamine (Prolactin Inhibiting Hormone) secreted from the hypothalamus. * **Insulin (Option A):** While insulin is an anabolic hormone, it does not serve as a primary stimulus for prolactin release. * **Growth Hormone (Option B):** GH and Prolactin are structurally similar (both are somatomammotropins), but GH does not stimulate the secretion of prolactin. **3. High-Yield NEET-PG Clinical Pearls:** * **TRH (Thyrotropin-Releasing Hormone):** In primary hypothyroidism, elevated TRH levels can stimulate lactotrophs, leading to **hyperprolactinemia**. * **Suckling Reflex:** The most important physiological stimulus for prolactin release post-partum. It inhibits dopamine release, thereby "disinhibiting" prolactin secretion. * **Prolactinoma:** The most common secretory tubulointerstitial tumor of the pituitary. * **Drugs:** Dopamine antagonists (e.g., Metoclopramide, Haloperidol) are common causes of drug-induced hyperprolactinemia.

Question 222: What is the major adrenocortical hormone in the blood?

A. Cortisol (Correct Answer)
B. Corticosterone
C. Aldosterone
D. Deoxycorticosterone

Explanation: **Explanation:** The adrenal cortex secretes several steroid hormones, but **Cortisol** is the predominant glucocorticoid and the most abundant adrenocortical hormone in human plasma. **1. Why Cortisol is the correct answer:** In humans, cortisol accounts for approximately **95% of all glucocorticoid activity**. It is secreted at a much higher rate (8–25 mg/day) compared to other steroids. Its high plasma concentration (average 10–20 µg/dL in the morning) and its critical role in glucose metabolism, stress response, and immune modulation make it the "major" hormone of the adrenal cortex. **2. Why the other options are incorrect:** * **Corticosterone (B):** While it has glucocorticoid activity, it is significantly less potent than cortisol in humans. It serves as a major glucocorticoid in rodents, but in humans, it is primarily a precursor in the aldosterone pathway. * **Aldosterone (C):** This is the primary mineralocorticoid. Although it is physiologically vital for sodium and potassium balance, its plasma concentration is extremely low (measured in **nanograms/dL**) compared to cortisol (measured in **micrograms/dL**). * **Deoxycorticosterone (D):** This is a precursor to aldosterone with mild mineralocorticoid activity. It is secreted in trace amounts and does not reach the systemic levels of cortisol. **High-Yield Facts for NEET-PG:** * **Transport:** 90–95% of cortisol is bound to **CBG (Transcortin)**; only the free fraction is biologically active. * **Diurnal Variation:** Cortisol levels peak in the early morning (approx. 8 AM) and are lowest around midnight. * **Potency:** Cortisol has a 1:1 ratio of glucocorticoid to mineralocorticoid activity, whereas Aldosterone has negligible glucocorticoid effect.

Question 223: Milk ejection is facilitated by:

A. Prolactin
B. Oxytocin (Correct Answer)
C. Growth hormone
D. ACTH

Explanation: **Explanation:** The correct answer is **Oxytocin**. **Mechanism of Milk Ejection:** Milk ejection (the "let-down reflex") is a neuroendocrine reflex. When an infant suckles, sensory receptors in the nipple send signals to the hypothalamus, specifically the **paraventricular and supraoptic nuclei**. This triggers the release of oxytocin from the **posterior pituitary** into the bloodstream. Oxytocin causes the contraction of **myoepithelial cells** surrounding the mammary alveoli, squeezing milk into the ductal system and out through the nipple. **Analysis of Incorrect Options:** * **A. Prolactin:** While essential for lactation, Prolactin is responsible for **milk production (synthesis)** within the alveolar cells, not its ejection. It is secreted by the anterior pituitary. * **C. Growth Hormone:** GH promotes general tissue growth and has a permissive role in mammary gland development, but it does not acutely trigger milk release. * **D. ACTH:** Secreted by the anterior pituitary, ACTH stimulates the adrenal cortex to release cortisol. It has no direct role in the milk ejection reflex. **High-Yield Clinical Pearls for NEET-PG:** * **The "Love Hormone":** Oxytocin also causes uterine contractions during labor (Ferguson reflex) and aids in uterine involution postpartum. * **Psychological Influence:** The milk ejection reflex can be conditioned; the sound of a baby crying can trigger oxytocin release, while stress or pain can inhibit it via catecholamines. * **Prolactin vs. Oxytocin:** Remember the mnemonic: **P**rolactin **P**roduces, **O**xytocin **O**ozes (ejects). * **Inhibition:** Prolactin secretion is tonically inhibited by **Dopamine** (Prolactin-inhibiting factor).

Question 224: Calcitriol causes all except?

A. Increases renal tubular reabsorption of Ca2+
B. Reduce plasma Ca2+ concentration (Correct Answer)
C. Promotes intestinal absorption of Ca2+
D. Reduces Ca2+ resorption by bone

Explanation: **Explanation:** The primary physiological role of **Calcitriol (1,25-dihydroxycholecalciferol)**, the active form of Vitamin D, is to **increase plasma calcium and phosphate levels**. Therefore, the statement that it reduces plasma calcium concentration is incorrect. **Why the correct answer is "Reduce plasma Ca2+ concentration":** Calcitriol acts as a hypercalcemic hormone. It maintains calcium homeostasis by increasing the influx of calcium into the extracellular fluid (ECF) from the gut, kidneys, and bone. Reducing plasma calcium is the function of **Calcitonin**, not Calcitriol. **Analysis of other options:** * **Option A (Increases renal reabsorption):** Calcitriol acts on the distal convoluted tubules of the kidney to increase the reabsorption of both Calcium and Phosphate, preventing their loss in urine. * **Option C (Promotes intestinal absorption):** This is the most important action of Calcitriol. It increases the synthesis of **Calbindin-D** (a calcium-binding protein) in intestinal epithelial cells, facilitating the active transport of Ca2+ from the diet. * **Option D (Reduces Ca2+ resorption by bone):** This is a nuanced point. While Calcitriol is necessary for bone mineralization (indirectly), its primary systemic effect at physiological levels is to work with PTH to mobilize calcium from bone (resorption) to maintain serum levels. However, in the context of this MCQ, the definitive "false" statement is the reduction of plasma calcium. **High-Yield NEET-PG Pearls:** * **Rate-limiting step:** The conversion of 25-OH Vitamin D to 1,25-(OH)₂ Vitamin D by the enzyme **1-alpha-hydroxylase** in the kidney. * **Stimulants for Calcitriol:** High PTH, low serum Calcium, and low serum Phosphate. * **Vitamin D Deficiency:** Leads to Rickets (children) and Osteomalacia (adults). * **Mnemonic:** Calci**tri**ol **tri**es to increase Calcium in **three** ways (Gut, Kidney, Bone).

Question 225: Which of the following describes a chemical messenger acting at long distances?

A. Autocrine
B. Paracrine
C. Endocrine (Correct Answer)
D. Auto-para-crine

Explanation: ### Explanation The classification of intercellular signaling is based on the distance the chemical messenger travels and the location of its target receptors. **Correct Answer: C. Endocrine** In **endocrine signaling**, specialized glands secrete hormones directly into the **bloodstream**. These chemical messengers travel through the systemic circulation to act on distant target cells throughout the body. Because the blood acts as the transport medium, endocrine signaling is uniquely characterized by its ability to function over **long distances**. Examples include Insulin (from the pancreas acting on muscle/fat) or TSH (from the pituitary acting on the thyroid). **Incorrect Options:** * **A. Autocrine:** The chemical messenger acts on the **same cell** that secreted it (e.g., Interleukin-2 in T-cell proliferation). * **B. Paracrine:** The messenger diffuses through the interstitial fluid to act on **neighboring/adjacent cells**. It does not enter the bloodstream and acts only over short distances (e.g., Histamine in inflammatory responses or Somatostatin acting within pancreatic islets). * **D. Auto-para-crine:** This is a hybrid term sometimes used to describe molecules that exhibit both properties, but it does not represent a messenger acting at long distances. **High-Yield Clinical Pearls for NEET-PG:** * **Neuroendocrine signaling:** A variation where neurons release messengers (neurohormones) into the blood (e.g., ADH from the posterior pituitary). * **Juxtacrine signaling:** Requires **direct contact** between cell membranes (e.g., Notch signaling). * **Speed of Action:** Endocrine signaling is generally slower than neural signaling but has a more prolonged and widespread effect.

Question 226: Which of the following would you expect to find in a patient whose diet has been low in calcium for 8 weeks?

A. Increased phosphate levels
B. Raised calcitonin levels
C. Increased parathormone secretion (Correct Answer)
D. Activation of 24,25-dihydroxycholecalciferol

Explanation: **Explanation:** The body maintains serum calcium within a very narrow range (8.5–10.5 mg/dL) through a tightly regulated feedback loop involving the parathyroid glands, kidneys, and bones. **1. Why Option C is Correct:** When dietary calcium intake is low for a prolonged period (8 weeks), the serum ionized calcium levels tend to fall. This hypocalcemia is sensed by **Calcium-Sensing Receptors (CaSR)** on the chief cells of the parathyroid gland. In response, the glands increase the synthesis and secretion of **Parathyroid Hormone (PTH)**. PTH acts to restore calcium levels by increasing bone resorption, enhancing renal calcium reabsorption, and stimulating the production of active Vitamin D. **2. Why the Other Options are Incorrect:** * **Option A (Increased phosphate levels):** PTH actually **decreases** serum phosphate levels. It inhibits the sodium-phosphate cotransporter in the proximal convoluted tubule of the kidney, leading to phosphaturia ("PTH thrashes phosphate"). * **Option B (Raised calcitonin levels):** Calcitonin is secreted by the parafollicular C-cells of the thyroid in response to *hypercalcemia* to lower calcium levels. In a low-calcium state, calcitonin secretion is suppressed. * **Option C (Activation of 24,25-dihydroxycholecalciferol):** This is an inactive metabolite. In the presence of high PTH (due to low calcium), the kidney activates the enzyme **1-alpha-hydroxylase** to produce **1,25-dihydroxycholecalciferol (Calcitriol)**, the active form of Vitamin D. 24,25-DHCC is produced only when calcium levels are normal or high. **NEET-PG High-Yield Pearls:** * **PTH** is the most important regulator of acute serum calcium levels. * **Vitamin D** is essential for the long-term absorption of dietary calcium from the intestine. * **Secondary Hyperparathyroidism:** A chronic low-calcium diet or Vitamin D deficiency leads to a compensatory increase in PTH, which can eventually lead to osteomalacia (in adults) or rickets (in children).

Question 227: Glucocorticoids have all the following actions except:

A. Increase in blood glucose level
B. Increase in protein catabolism
C. Anti-insulin action in peripheral tissues
D. Decrease in glucose uptake by the heart (Correct Answer)

Explanation: **Explanation:** Glucocorticoids (primarily Cortisol) are "stress hormones" that function to increase the availability of metabolic fuels. The core physiological principle is to prioritize glucose delivery to vital organs (Brain and Heart) while limiting its use in peripheral tissues. **Why Option D is the Correct Answer:** Glucocorticoids induce peripheral insulin resistance to conserve glucose. However, this inhibitory effect on glucose uptake occurs in **skeletal muscle and adipose tissue** (via inhibition of GLUT-4 translocation). Crucially, glucocorticoids **do not decrease glucose uptake in the heart or the brain**. In fact, during stress, the heart's metabolic demands are maintained or increased, making Option D a false statement regarding glucocorticoid action. **Analysis of Incorrect Options:** * **Option A (Increase in blood glucose):** Correct action. Cortisol stimulates **gluconeogenesis** in the liver and decreases peripheral utilization, leading to hyperglycemia (Adrenal diabetes). * **Option B (Increase in protein catabolism):** Correct action. Glucocorticoids are catabolic in extra-hepatic tissues (muscle, lymphoid, and connective tissue) to provide amino acids for hepatic gluconeogenesis. * **Option C (Anti-insulin action):** Correct action. They antagonize insulin's effects in peripheral tissues by interfering with the insulin signaling pathway, leading to decreased glucose uptake in muscles. **High-Yield NEET-PG Pearls:** * **Permissive Action:** Small amounts of glucocorticoids are required for catecholamines to exert their calorigenic and lipolytic effects. * **Hematological effects:** They cause **"Lymphocytopenia"** and **"Eosinopenia"** but lead to **"Neutrophilia"** (due to decreased margination). * **Bone:** They inhibit osteoblast activity and decrease intestinal Calcium absorption, leading to osteoporosis in Cushing’s syndrome.

Question 228: Which of the following hormones is produced by the anterior lobe of the pituitary gland?

A. Thyrotropin-releasing hormone (TRH)
B. Corticotropin-releasing hormone (CRH)
C. Growth hormone (somatotropin, GH) (Correct Answer)
D. Growth hormone-releasing hormone (GHRH)

Explanation: **Explanation:** The pituitary gland is divided into the anterior lobe (adenohypophysis) and the posterior lobe (neurohypophysis). The **Anterior Pituitary** is a true endocrine gland that synthesizes and secretes six major peptide hormones: Growth Hormone (GH), Adrenocorticotropic hormone (ACTH), Thyroid-stimulating hormone (TSH), Follicle-stimulating hormone (FSH), Luteinizing hormone (LH), and Prolactin (PRL). **Growth Hormone (GH)** is produced by the **somatotroph cells** of the anterior pituitary. Its secretion is regulated by the hypothalamus via GH-releasing hormone (stimulatory) and Somatostatin (inhibitory). **Analysis of Incorrect Options:** * **Options A, B, and D (TRH, CRH, and GHRH):** These are all **Hypothalamic hormones**. The hypothalamus produces "releasing" and "inhibiting" hormones that travel through the hypophyseal portal system to regulate the secretion of hormones from the anterior pituitary. A simple rule for NEET-PG: if the hormone name ends in "-releasing hormone," its origin is the **Hypothalamus**, not the pituitary. **High-Yield Facts for NEET-PG:** * **Embryology:** The anterior pituitary develops from **Rathke’s pouch** (oral ectoderm), while the posterior pituitary develops from the **infundibulum** (neuroectoderm). * **Posterior Pituitary:** It does *not* synthesize hormones; it only stores and releases **Oxytocin** and **ADH (Vasopressin)**, which are synthesized in the supraoptic and paraventricular nuclei of the hypothalamus. * **Acidophils vs. Basophils:** In the anterior pituitary, GH and Prolactin are secreted by **acidophils**, while TSH, ACTH, FSH, and LH are secreted by **basophils** (Mnemonic: **B-FLAT** for Basophils).

Question 229: What is the ratio of C-peptide to insulin in normal adults?

A. 2:1
B. 1:2
C. 1:1 (Correct Answer)
D. 1:4

Explanation: ### Explanation **Concept and Correct Answer:** The synthesis of insulin occurs in the pancreatic beta cells. It begins as **Preproinsulin**, which is cleaved into **Proinsulin**. Proinsulin is then packaged into secretory granules, where it undergoes proteolytic cleavage by endopeptidases. This process splits the proinsulin molecule into two distinct parts: one molecule of **Insulin** and one molecule of **C-peptide** (Connecting peptide). Because they are derived from the same precursor molecule and released together via exocytosis in a stoichiometric fashion, they are secreted into the portal circulation in a **1:1 molar ratio**. Therefore, **Option C** is the correct answer. **Analysis of Incorrect Options:** * **Option A (2:1) and B (1:2):** These ratios are incorrect because the cleavage of one proinsulin molecule cannot mathematically yield more of one component than the other. * **Option D (1:4):** This does not reflect the physiological production process. While peripheral blood levels may show higher C-peptide concentrations due to different half-lives, the initial secretion ratio remains 1:1. **High-Yield Clinical Pearls for NEET-PG:** * **Half-life:** Although secreted 1:1, C-peptide has a longer half-life (~30 mins) compared to insulin (~5 mins). Consequently, peripheral plasma levels of C-peptide are typically higher than insulin. * **Metabolism:** Insulin undergoes significant first-pass metabolism in the **liver**, whereas C-peptide is primarily excreted by the **kidneys**. * **Diagnostic Utility:** C-peptide is a reliable marker of **endogenous insulin production**. It is used to distinguish between Type 1 and Type 2 Diabetes and to differentiate Factitious Hypoglycemia (low C-peptide) from Insulinoma (high C-peptide).

Question 230: 25-dihydrocholecalciferol acts on which type of receptors?

A. Surface receptors
B. Cytosolic receptors
C. Intranuclear receptors (Correct Answer)
D. None of the above

Explanation: **Explanation:** The correct answer is **C. Intranuclear receptors.** **Mechanism of Action:** 1,25-dihydroxycholecalciferol (Calcitriol), the active form of Vitamin D, is a steroid-like hormone derived from cholesterol. Due to its lipophilic nature, it easily crosses the lipid bilayer of the cell membrane. Once inside the cell, it enters the nucleus and binds to the **Vitamin D Receptor (VDR)**. The VDR then forms a heterodimer with the **Retinoid X Receptor (RXR)**. This complex binds to specific DNA sequences known as **Vitamin D Response Elements (VDREs)**, initiating gene transcription and the synthesis of proteins like Calbindin, which facilitates intestinal calcium absorption. **Analysis of Options:** * **A. Surface receptors:** These are typically for peptide hormones (e.g., Insulin, PTH) or catecholamines that cannot cross the cell membrane. Vitamin D is lipid-soluble and does not require surface signaling. * **B. Cytosolic receptors:** While some steroid hormones (like Glucocorticoids) initially bind in the cytosol before translocating to the nucleus, the Vitamin D receptor is primarily localized within the **nucleus** even in its unoccupied state. **High-Yield Clinical Pearls for NEET-PG:** * **Target Organs:** The primary sites of action for Vitamin D are the Intestine (increases Ca/PO4 absorption), Bone (mineralization/resorption), and Kidney (increases Ca/PO4 reabsorption). * **Steroid Superfamily:** Vitamin D, Thyroid hormones (T3/T4), Retinoic acid, and Steroid hormones all act via nuclear receptors. * **Rate-limiting step:** The conversion of 25-OH Vitamin D to 1,25-(OH)₂ Vitamin D occurs in the kidney via the enzyme **1-alpha-hydroxylase**, which is stimulated by PTH.

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

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Hormone Receptors and Signaling

Practice Questions

Assessment of Endocrine Function

Practice Questions

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