Endocrinology — MCQs

Endocrinology Indian Medical PG Practice Questions and MCQs

Question 421: Which of the following is a function of prolactin during lactation?

A. Increased FSH
B. Increased libido
C. Increased testosterone
D. Decreased estrogen (Correct Answer)

Explanation: **Explanation:** The correct answer is **D. Decreased estrogen**. **Mechanism:** Prolactin, primarily known for stimulating milk production (lactogenesis), exerts a potent inhibitory effect on the hypothalamic-pituitary-gonadal (HPG) axis. High levels of prolactin during lactation inhibit the pulsatile release of **Gonadotropin-Releasing Hormone (GnRH)** from the hypothalamus. This suppression leads to a decrease in the secretion of **FSH (Follicle-Stimulating Hormone)** and **LH (Luteinizing Hormone)** from the anterior pituitary. Consequently, the ovaries are not stimulated to produce **estrogen**, leading to a state of temporary hypogonadism known as **lactational amenorrhea**. **Analysis of Incorrect Options:** * **A. Increased FSH:** Prolactin suppresses GnRH, which leads to a *decrease* in FSH levels, not an increase. * **B. Increased libido:** Estrogen and testosterone are key drivers of libido. Since prolactin suppresses these sex steroids, it typically results in *decreased* libido. * **C. Increased testosterone:** In both males and females, hyperprolactinemia inhibits the HPG axis, leading to *decreased* production of testosterone. **Clinical Pearls for NEET-PG:** * **Lactational Amenorrhea Method (LAM):** This physiological suppression of ovulation serves as a natural (though not 100% reliable) form of contraception postpartum. * **Prolactinoma:** In non-lactating patients, a prolactin-secreting tumor presents with galactorrhea, infertility, and decreased bone density (due to chronic low estrogen). * **Dopamine Connection:** Dopamine is the primary "Prolactin-Inhibiting Factor." Drugs that block dopamine (like antipsychotics) can cause hyperprolactinemia.

Question 422: Pheochromocytoma predominantly secretes which hormone?

A. Serotonin
B. Epinephrine
C. Norepinephrine (Correct Answer)
D. Dopamine

Explanation: ### Explanation **Pheochromocytoma** is a catecholamine-secreting tumor derived from the chromaffin cells of the adrenal medulla. **Why Norepinephrine is Correct:** In the normal adrenal medulla, approximately 80% of the secretion is Epinephrine and 20% is Norepinephrine. This is because the enzyme **PNMT** (Phenylethanolamine N-methyltransferase), which converts norepinephrine to epinephrine, is induced by high concentrations of cortisol draining from the adrenal cortex. However, in a **Pheochromocytoma**, the tumor cells often lack sufficient contact with cortical cortisol or lose the ability to express PNMT efficiently. Consequently, the predominant hormone secreted into the circulation is **Norepinephrine**. **Analysis of Incorrect Options:** * **A. Serotonin:** This is primarily secreted by enterochromaffin cells of the GI tract and is the hallmark of Carcinoid Syndrome, not Pheochromocytoma. * **B. Epinephrine:** While secreted by the normal adrenal medulla and some specific pheochromocytomas (especially those associated with MEN 2 syndrome), it is not the *predominant* secretion in sporadic cases. * **D. Dopamine:** Though a precursor in the catecholamine pathway, it is rarely the primary secretion. High dopamine levels are more characteristic of neuroblastomas or extra-adrenal paragangliomas. **High-Yield Clinical Pearls for NEET-PG:** * **Rule of 10s:** 10% bilateral, 10% malignant, 10% pediatric, 10% extra-adrenal (Paraganglioma). * **Diagnosis:** Best initial screening test is **24-hour urinary fractionated metanephrines** and VMA. * **Management:** Always give **Alpha-blockers (Phenoxybenzamine)** before Beta-blockers to prevent a hypertensive crisis (unopposed alpha-1 stimulation). * **Associated Syndromes:** MEN 2A, MEN 2B, von Hippel-Lindau (VHL), and NF-1.

Question 423: Calcitonin is released by:

A. Parafollicular cells of thyroid (Correct Answer)
B. Chief cells of thyroid
C. Granular cells of adrenal gland
D. Stratum fasciculata of adrenal gland

Explanation: **Explanation:** **Calcitonin** is a 32-amino acid peptide hormone primarily involved in calcium homeostasis. It is synthesized and secreted by the **Parafollicular cells (also known as C-cells)** of the thyroid gland. These cells are neuroendocrine in origin (derived from the neural crest) and are located in the connective tissue between the thyroid follicles. Calcitonin acts to lower plasma calcium levels by inhibiting osteoclast activity and decreasing renal calcium reabsorption, effectively acting as a physiological antagonist to Parathyroid Hormone (PTH). **Analysis of Incorrect Options:** * **B. Chief cells of thyroid:** This is a distractor. Chief cells are found in the **Parathyroid glands**, where they secrete Parathyroid Hormone (PTH), not in the thyroid gland. * **C. Granular cells of adrenal gland:** There are no specific "granular cells" defined as a major secretory unit in the adrenal gland. The adrenal medulla contains Chromaffin cells which secrete catecholamines. * **D. Stratum fasciculata of adrenal gland:** This is the middle layer of the adrenal cortex responsible for secreting **Glucocorticoids** (primarily Cortisol), not calcitonin. **High-Yield Clinical Pearls for NEET-PG:** * **Stimulus for Release:** The primary stimulus for calcitonin secretion is **hypercalcemia**. * **Tumor Marker:** Calcitonin serves as a critical tumor marker for **Medullary Thyroid Carcinoma (MTC)**, which arises from the Parafollicular C-cells. * **Therapeutic Use:** Exogenous calcitonin (often salmon calcitonin) is used clinically to treat Paget’s disease of the bone and severe hypercalcemia. * **Developmental Origin:** While thyroid follicular cells arise from the endoderm (thyroglossal duct), the Parafollicular C-cells originate from the **ultimobranchial body** (derived from the 4th/5th pharyngeal pouches).

Question 424: Normally, Cortisol levels are maximum at __ and lowest at ?

A. 8 pm and early morning
B. 12 pm and 12 am
C. 8 am and 8 pm
D. 8 am and midnight (Correct Answer)

Explanation: ### Explanation **Concept: Circadian Rhythm of Cortisol** The secretion of Cortisol follows a distinct **circadian (diurnal) rhythm**, which is regulated by the suprachiasmatic nucleus (SCN) of the hypothalamus. This rhythm is synchronized with the sleep-wake cycle and exposure to light. 1. **Peak (Maximum):** Cortisol levels begin to rise during the late stages of sleep and reach their **peak around 8:00 AM**. This "morning surge" prepares the body for the stresses of the day by increasing blood glucose and vascular reactivity. 2. **Nadir (Lowest):** Levels gradually decline throughout the day, reaching their **lowest point (nadir) around midnight** (usually 1–2 hours after the onset of sleep). **Analysis of Options:** * **Option D (Correct):** Accurately reflects the physiological peak (8 am) and nadir (midnight) observed in individuals with a standard sleep-wake cycle. * **Option A:** Incorrectly reverses the pattern; 8 pm is near the decline, and early morning is when levels are rising, not at their lowest. * **Option B:** 12 pm (noon) shows declining levels, and while 12 am is the nadir, the peak occurs earlier than noon. * **Option C:** While 8 am is the peak, 8 pm is not the lowest point; levels continue to drop further until midnight. **High-Yield Clinical Pearls for NEET-PG:** * **Cushing Syndrome:** The earliest biochemical sign is the **loss of diurnal variation** (elevated late-night salivary or serum cortisol). * **Diagnostic Timing:** To diagnose adrenal insufficiency (Addison’s), cortisol is measured at **8 am** (when it should be high). To screen for Cushing syndrome, it is measured at **midnight** (when it should be low). * **Stress Factor:** This rhythm can be abolished by physical or psychological stress, which causes persistent ACTH elevation. * **Blindness:** In totally blind individuals, the rhythm may "free-run" (shift) because the SCN lacks light-dark synchronization.

Question 425: Which of the following is NOT an action of calcitriol?

A. Calcitriol increases the intestinal absorption of calcium.
B. Calcitriol stimulates calcium uptake for deposition as calcium phosphate in the osteoblasts of bone.
C. Calcitriol increases the excretion of calcium and phosphate through the kidney. (Correct Answer)
D. Calcitriol's action is similar to steroid hormones.

Explanation: **Explanation** Calcitriol ($1,25$-dihydroxycholecalciferol) is the active form of Vitamin D. Its primary physiological role is to **increase plasma calcium and phosphate levels** to ensure adequate mineralization of bone. **Why Option C is the correct answer:** Calcitriol does **not** increase the excretion of calcium and phosphate. Instead, it acts on the distal renal tubules to **increase the reabsorption** of both calcium and phosphate from the glomerular filtrate back into the blood. By reducing their loss in urine, calcitriol helps maintain positive mineral balance. **Analysis of Incorrect Options:** * **Option A:** This is a primary action. Calcitriol induces the synthesis of **Calbindin-D** (a calcium-binding protein) in intestinal epithelial cells, which facilitates the active transport of dietary calcium. * **Option B:** While high doses of calcitriol can cause bone resorption (via RANKL), its physiological role at normal levels is to provide sufficient calcium and phosphate for **osteoblastic activity** and bone mineralization. * **Option D:** Calcitriol is a steroid-like hormone. It crosses the cell membrane and binds to a **nuclear receptor (VDR)**, which then binds to DNA to regulate gene transcription, similar to cortisol or aldosterone. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting step:** The conversion of $25(OH)D_3$ to $1,25(OH)_2D_3$ by the enzyme **$1\alpha$-hydroxylase** in the kidney (stimulated by PTH). * **Synergy:** Calcitriol and PTH act synergistically to increase bone resorption to maintain serum calcium during deficiency. * **Deficiency:** Leads to **Rickets** in children (failure of mineralization at growth plates) and **Osteomalacia** in adults.

Question 426: Glucose-dependent release of insulin is mediated by which mechanism?

A. Cyclic AMP (Correct Answer)
B. Carrier's modulators
C. Receptor phosphorylation
D. ATP-dependent K+ channel

Explanation: **Explanation:** The release of insulin from pancreatic beta cells occurs in two distinct phases: the **triggering pathway** and the **amplifying pathway**. **Why Option A is Correct:** While the ATP-sensitive K+ channel initiates the process, the **glucose-dependent amplification** of insulin secretion is primarily mediated by **Cyclic AMP (cAMP)**. When glucose levels rise, it stimulates the production of cAMP. cAMP acts via two main targets: **Protein Kinase A (PKA)** and **Epac2** (Exchange protein directly activated by cAMP). These molecules increase the size of the "readily releasable pool" of insulin vesicles and enhance the efficiency of exocytosis. Furthermore, Incretins (like GLP-1), which are secreted in response to oral glucose, exert their potent insulinotropic effect specifically by increasing intracellular cAMP levels. **Analysis of Incorrect Options:** * **Option B (Carrier's modulators):** This is a non-specific term and does not represent a recognized physiological signaling pathway for insulin secretion. * **Option C (Receptor phosphorylation):** This describes the **action** of insulin on target tissues (via its tyrosine kinase receptor) rather than the mechanism of its **release** from the pancreas. * **Option D (ATP-dependent K+ channel):** This is the "triggering" mechanism. While essential for depolarization, it is considered the threshold step. In the context of "glucose-dependent" enhancement and the potentiation of the insulin response (the "Incretin effect"), cAMP is the key mediator. **High-Yield Clinical Pearls for NEET-PG:** * **Incretin Effect:** Oral glucose causes a much higher insulin release than IV glucose due to GLP-1 and GIP, which act via the **cAMP pathway**. * **Sulfonylureas:** These drugs bypass the glucose requirement by directly closing the **ATP-sensitive K+ channels**, leading to depolarization and insulin release. * **MODY Type 2:** Caused by a mutation in **Glucokinase**, the "glucose sensor" of the beta cell.

Question 427: Insulin acts by stimulation of which type of receptor?

A. Ionotropic receptor
B. Enzymatic receptor (Correct Answer)
C. Metabotropic receptor
D. Nuclear receptor

Explanation: **Explanation:** The insulin receptor belongs to the **Enzymatic receptor** family, specifically the **Receptor Tyrosine Kinase (RTK)** class. It is a heterotetramer consisting of two extracellular alpha-subunits (binding site) and two transmembrane beta-subunits. When insulin binds to the alpha-subunits, it triggers autophosphorylation of the beta-subunits. This activates the intrinsic tyrosine kinase activity, which then phosphorylates **Insulin Receptor Substrates (IRS)**, leading to downstream metabolic effects like glucose uptake via GLUT-4 translocation. **Analysis of Options:** * **A. Ionotropic receptors:** These are ligand-gated ion channels (e.g., Nicotinic ACh receptors) that change membrane potential. Insulin does not directly open an ion channel. * **C. Metabotropic receptors:** These are G-Protein Coupled Receptors (GPCRs) that act via second messengers like cAMP or IP3/DAG (e.g., Glucagon, Epinephrine). Insulin does not utilize G-proteins. * **D. Nuclear receptors:** These are intracellular receptors for lipid-soluble hormones (e.g., Steroids, Thyroid hormone) that act as transcription factors. Insulin is a peptide hormone and cannot cross the lipid bilayer. **High-Yield Clinical Pearls for NEET-PG:** * **GLUT-4:** The only insulin-dependent glucose transporter, found primarily in skeletal muscle and adipose tissue. * **MAP Kinase Pathway:** Mediates the growth-promoting (mitogenic) effects of insulin. * **PI3-Kinase Pathway:** Mediates the metabolic effects (glucose transport and glycogen synthesis). * **Downregulation:** Chronic hyperinsulinemia (as seen in Type 2 Diabetes) leads to "internalization" of receptors, contributing to insulin resistance.

Question 428: In a transection between the hypothalamus and the anterior pituitary, the production of which hormone remains unaffected?

A. Testosterone
B. Oxytocin (Correct Answer)
C. Thyroxine
D. Cortisol

Explanation: ### Explanation The key to answering this question lies in understanding the anatomical and functional connection between the hypothalamus and the pituitary gland. **Why Oxytocin is the Correct Answer:** Oxytocin and Vasopressin (ADH) are synthesized in the **magnocellular neurons** of the paraventricular and supraoptic nuclei of the **hypothalamus**. These hormones are then transported via the axons of the hypothalamo-hypophyseal tract to the **posterior pituitary** (neurohypophysis), where they are stored and released into the systemic circulation. A transection between the hypothalamus and the pituitary (pituitary stalk) disrupts the portal blood flow but does not stop the *production* of oxytocin in the hypothalamus. While its *release* into the blood may be acutely impaired, the synthesis remains intact within the hypothalamic nuclei. **Why the Other Options are Incorrect:** * **Testosterone, Thyroxine, and Cortisol:** These hormones are part of the hypothalamic-pituitary-peripheral gland axes. Their production depends on **releasing hormones** (GnRH, TRH, and CRH) secreted by the hypothalamus into the **hypophyseal portal system**. A transection severs these portal vessels, preventing the releasing hormones from reaching the **anterior pituitary**. This leads to a cessation of trophic hormone secretion (LH/FSH, TSH, and ACTH), ultimately resulting in the failure of the peripheral glands to produce testosterone, thyroxine, and cortisol. **High-Yield Clinical Pearls for NEET-PG:** * **The "Stalk Section" Effect:** Following a pituitary stalk transection, the levels of all anterior pituitary hormones decrease **except Prolactin**. Prolactin levels rise because the tonic inhibitory influence of hypothalamic **Dopamine** (Prolactin-Inhibiting Hormone) is lost. * **Diabetes Insipidus:** While ADH/Oxytocin production continues in the hypothalamus after a low stalk transection, permanent Diabetes Insipidus only occurs if the lesion is high enough to cause retrograde neuronal degeneration of the hypothalamic nuclei. * **Master Gland:** Remember that the posterior pituitary does not synthesize any hormones; it is merely a storage site.

Question 429: What causes the increase in endometrial thickness during the post-ovulatory phase?

A. Progesterone (Correct Answer)
B. Estrogen
C. Follicle-Stimulating Hormone (FSH)
D. Luteinizing Hormone (LH)

Explanation: ### **Explanation** The endometrial cycle is divided into the proliferative phase and the secretory phase. The question refers to the **post-ovulatory phase**, also known as the **Secretory Phase**. **1. Why Progesterone is Correct:** After ovulation, the ruptured follicle transforms into the **Corpus Luteum**, which primarily secretes **Progesterone**. While estrogen causes the initial growth (proliferation), Progesterone is responsible for the further thickening and "maturation" of the endometrium. It induces tortuosity of the endometrial glands, increases vascularity (spiral arteries), and promotes the accumulation of glycogen and lipids. This prepares the lining for the potential implantation of a blastocyst. **2. Why the Other Options are Incorrect:** * **Estrogen:** This is the dominant hormone of the **pre-ovulatory (Proliferative) phase**. It causes the initial regeneration of the endometrium after menstruation by increasing the number of cells (hyperplasia). * **FSH (Follicle-Stimulating Hormone):** Secreted by the anterior pituitary, its primary role is to stimulate the growth of ovarian follicles during the follicular phase. It does not have a direct effect on endometrial thickness. * **LH (Luteinizing Hormone):** The LH surge triggers **ovulation**. While LH maintains the corpus luteum (which then produces progesterone), it is not the direct effector hormone for endometrial changes. ### **High-Yield NEET-PG Pearls:** * **Predominant hormone of Secretory Phase:** Progesterone. * **Predominant hormone of Proliferative Phase:** Estrogen. * **Histological hallmark of the Secretory Phase:** Sub-nuclear vacuolation (the first sign of progesterone effect, appearing ~36–48 hours after ovulation). * **Spiral Arteries:** These develop specifically under the influence of progesterone; their constriction due to progesterone withdrawal leads to menstruation.

Question 430: Hung up reflex is seen in which of the following conditions?

A. Myxedema
B. Hyperthyroidism
C. Hypothyroidism (Correct Answer)
D. Pheochromocytoma

Explanation: **Explanation:** The **"Hung-up reflex"** (also known as Woltman’s sign) refers to the delayed relaxation phase of a deep tendon reflex, most commonly elicited in the Achilles tendon. **1. Why Hypothyroidism is correct:** In hypothyroidism, there is a generalized slowing of metabolic processes. The delayed relaxation is not due to a defect in the nerve conduction or the initial contraction phase, but rather a **slowing of the calcium reuptake** by the sarcoplasmic reticulum and a decrease in the rate of cross-bridge cycling in skeletal muscle. This results in a characteristic "slow return" of the limb to its neutral position. While "Myxedema" (Option A) is a severe form of hypothyroidism, "Hypothyroidism" is the broader, standard clinical term used in examinations for this sign. **2. Why other options are incorrect:** * **Hyperthyroidism:** This condition presents with **brisk** or hyperactive reflexes (shortened contraction and relaxation phases) due to increased neuromuscular excitability. * **Pheochromocytoma:** This catecholamine-secreting tumor leads to hypertension and tachycardia. While it may cause tremors or hyperreflexia due to sympathetic overactivity, it does not cause a delayed relaxation phase. **3. Clinical Pearls for NEET-PG:** * **Woltman’s Sign:** Another name for the hung-up reflex; it is considered a highly specific clinical sign for hypothyroidism. * **Other causes of delayed relaxation:** Besides hypothyroidism, it can occasionally be seen in pregnancy, diabetes mellitus, hypothermia, and with the use of beta-blockers. * **Muscle involvement in Hypothyroidism:** Look out for **Hoffmann’s Syndrome** (pseudohypertrophy of muscles associated with hypothyroidism in adults) and **Kocher-Debre-Semelaigne Syndrome** (similar presentation in children).

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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