Endocrinology — MCQs

Endocrinology Indian Medical PG Practice Questions and MCQs

Question 541: A high parathyroid hormone (PTH) level affects which of the following processes to return plasma calcium levels toward normal?

A. Inhibit calcium secretion from the gastrointestinal tract
B. Reduce the expression of plasma calcium-binding proteins
C. Stimulate bone resorption, leading to the release of calcium into the plasma (Correct Answer)
D. Stimulate the release of calcium from muscle cells

Explanation: **Explanation:** Parathyroid Hormone (PTH) is the primary regulator of plasma calcium homeostasis. When serum ionized calcium levels drop, the parathyroid glands release PTH to elevate calcium through three main target organs: bone, kidneys, and the gastrointestinal tract. **Why Option C is Correct:** PTH directly stimulates osteoblasts to release **RANK-ligand (RANKL)** and **M-CSF**, which in turn activate **osteoclasts**. These osteoclasts promote bone resorption, breaking down the hydroxyapatite matrix and releasing calcium and phosphate into the plasma. This is the most immediate and potent mechanism by which PTH increases plasma calcium. **Why Other Options are Incorrect:** * **Option A:** PTH does not inhibit calcium secretion; instead, it indirectly **increases calcium absorption** from the gut by stimulating the renal enzyme **1-alpha-hydroxylase**, which converts Vitamin D to its active form, Calcitriol. * **Option B:** Plasma calcium-binding proteins (like albumin) are primarily influenced by nutritional status and pH, not by PTH levels. * **Option D:** Calcium in muscle cells is sequestered in the sarcoplasmic reticulum for contraction-relaxation cycles and is not a reservoir used for systemic plasma calcium regulation. **High-Yield NEET-PG Pearls:** * **Renal Action:** PTH increases calcium reabsorption in the **Distal Convoluted Tubule (DCT)** but decreases phosphate reabsorption in the **Proximal Convoluted Tubule (PCT)** (causing phosphaturia). * **Bone Paradox:** While continuous high PTH (hyperparathyroidism) causes bone resorption, **intermittent low doses** of PTH (e.g., Teriparatide) actually stimulate bone formation (anabolic effect). * **Mnemonic:** PTH = **P**hosphate **T**hrashing **H**ormone (it "trashes" phosphate in the urine to raise calcium).

Question 542: Which of the following is NOT a prolactin-releasing factor?

A. Vasopressin
B. Vasoactive intestinal peptide
C. Dopamine (Correct Answer)
D. Acetylcholine

Explanation: ### Explanation The regulation of prolactin (PRL) secretion is unique among anterior pituitary hormones because it is under **tonic inhibition** by the hypothalamus. **1. Why Dopamine is the Correct Answer:** Dopamine is the primary **Prolactin-Inhibiting Factor (PIF)**. It is secreted by the tuberoinfundibular dopaminergic (TIDA) neurons into the hypophyseal portal system. It acts on **D2 receptors** located on the lactotrophs of the anterior pituitary to decrease cAMP levels, thereby inhibiting the synthesis and release of prolactin. Therefore, it is not a releasing factor but a potent inhibitor. **2. Analysis of Other Options (Releasing Factors):** While there is no single "Prolactin Releasing Hormone," several substances stimulate its release: * **Vasoactive Intestinal Peptide (VIP):** A potent stimulator of prolactin release, often acting in a paracrine fashion within the pituitary. * **Vasopressin (ADH):** Known to stimulate lactotrophs, especially during stress responses. * **Acetylcholine:** Acts via muscarinic receptors to stimulate prolactin secretion (though its role is less dominant than TRH or VIP). * *Note:* **TRH (Thyrotropin-Releasing Hormone)** is also a major physiological prolactin-releasing factor. **3. NEET-PG High-Yield Pearls:** * **The "Disconnect" Effect:** Any injury to the pituitary stalk (trauma, tumors like Craniopharyngioma) leads to a decrease in all pituitary hormones **EXCEPT prolactin**, which rises because it is freed from dopamine's inhibitory control. * **Drug-Induced Hyperprolactinemia:** Antipsychotics (D2 antagonists) like Haloperidol or Risperidone commonly cause galactorrhea by blocking dopamine's inhibitory effect. * **Prolactin & Amenorrhea:** High prolactin levels inhibit **GnRH pulsatility**, leading to secondary amenorrhea and infertility.

Question 543: Plasma half-life of insulin is:

A. 1 minute
B. 10 minutes (Correct Answer)
C. 1 hour
D. 2 hours

Explanation: **Explanation:** The plasma half-life of insulin is a high-yield fact in endocrinology. Once secreted into the portal circulation, insulin is rapidly cleared from the plasma, primarily by the liver (about 50%) and the kidneys. **1. Why 10 minutes is correct:** In healthy individuals, the biological half-life of endogenous insulin is approximately **5 to 10 minutes**. This rapid turnover is physiological; it allows the body to make quick, minute-to-minute adjustments in blood glucose levels. If insulin persisted longer, post-prandial glucose regulation would be imprecise, leading to prolonged hypoglycemia after a meal. **2. Why the other options are incorrect:** * **1 minute (Option A):** This is too short. While insulin action begins quickly, the degradation process by the enzyme *insulinase* takes slightly longer than sixty seconds to reduce plasma concentration by half. * **1 hour (Option C) and 2 hours (Option D):** These durations are far too long for endogenous insulin. However, students often confuse this with the **pharmacokinetics of exogenous "Regular" insulin**, which has a peak action at 2–3 hours and a duration of 5–8 hours when injected subcutaneously. **Clinical Pearls for NEET-PG:** * **Degradation:** The primary enzyme responsible for insulin breakdown is **Insulin-degrading enzyme (IDE)** or insulinase, found mainly in the liver and kidneys. * **C-Peptide:** Unlike insulin, C-peptide has a longer half-life (approx. **30 minutes**). This makes it a better clinical marker for measuring endogenous insulin production (beta-cell function). * **Excretion:** The liver clears the majority of endogenous insulin, but the **kidneys** clear the majority of exogenous (injected) insulin. Therefore, insulin requirements often decrease in patients with chronic kidney disease (CKD).

Question 544: Which of the following hormones is mainly responsible for skeletal maturation?

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

Explanation: **Explanation:** The correct answer is **Testosterone**. **1. Why Testosterone is correct:** In males, **Testosterone** is the primary driver of skeletal maturation and the pubertal growth spurt. While Growth Hormone (GH) increases bone length, testosterone (and its aromatization to estrogen) is essential for increasing bone mineral density and the eventual **fusion of the epiphyseal plates**. It promotes the maturation of chondrocytes into osteoblasts, leading to the mineralization of the growth plate, which terminates linear growth. **2. Why the other options are incorrect:** * **Estrogen:** While estrogen is the key hormone for epiphyseal closure in *both* sexes, the question asks for the hormone "mainly responsible" in a general physiological context or specifically in males if implied. However, in many medical entrance exams (like NEET-PG), testosterone is traditionally cited as the primary androgenic driver for skeletal maturation in males. *Note: If the question specified females or "epiphyseal closure" specifically, estrogen would be the most potent factor.* * **Growth Hormone (GH):** GH is primarily responsible for **linear bone growth** (length) via IGF-1, but it does not cause skeletal maturation (fusion) on its own. In fact, GH deficiency leads to delayed skeletal age, but GH excess does not necessarily accelerate maturation. * **Insulin:** Insulin has a permissive effect on growth and is structurally similar to IGF-1, but it plays no direct role in the maturation or fusion of the skeleton. **Clinical Pearls for NEET-PG:** * **Precocious Puberty:** Early exposure to sex steroids (testosterone/estrogen) leads to an initial growth spurt but results in **short stature** due to premature skeletal maturation and epiphyseal fusion. * **Hypogonadism:** A deficiency in testosterone leads to delayed skeletal maturation, resulting in "eunuchoid proportions" (long limbs because the epiphyses stay open longer). * **Thyroid Hormone:** Also essential for bone maturation; its deficiency causes significant "bone age" delay.

Question 545: Which of the following is caused by congenital A-17 hydroxylase deficiency?

A. Hypercalcemia
B. Hyperkalemia
C. Hermaphroditism
D. Hypertension (Correct Answer)

Explanation: **Explanation:** Congenital 17α-hydroxylase deficiency is a rare form of Congenital Adrenal Hyperplasia (CAH) characterized by a block in the conversion of pregnenolone and progesterone into their 17-hydroxy derivatives. This leads to a specific shift in the steroidogenic pathway. **Why Hypertension is the Correct Answer:** The enzyme block prevents the synthesis of cortisol and sex hormones (androgens/estrogens). To compensate for low cortisol, ACTH levels rise via negative feedback. This stimulates the pathway "upstream" of the block, leading to the overproduction of **11-deoxycorticosterone (DOC)** and **corticosterone**. DOC is a potent mineralocorticoid; its excess causes sodium retention and volume expansion, resulting in **hypertension** and secondary suppression of renin and aldosterone. **Analysis of Incorrect Options:** * **Hypercalcemia:** Calcium metabolism is regulated by PTH and Vitamin D; it is not directly affected by adrenal steroid enzyme deficiencies. * **Hyperkalemia:** In 17α-hydroxylase deficiency, the excess mineralocorticoids (DOC) promote potassium excretion in the distal tubule, leading to **hypokalemia**, not hyperkalemia. (Hyperkalemia is seen in 21-hydroxylase deficiency). * **Hermaphroditism:** Because the deficiency prevents androgen production, genetic males (46,XY) present with female external genitalia or ambiguous genitalia (pseudohermaphroditism), while genetic females (46,XX) fail to develop secondary sexual characteristics (delayed puberty). True hermaphroditism involves the presence of both ovarian and testicular tissue, which is not the mechanism here. **High-Yield Clinical Pearls for NEET-PG:** * **The "Rule of 1":** If the enzyme starts with **1** (11, 17), it causes hypertension. * **17α-hydroxylase deficiency mnemonic:** "High BP, Low Sex" (Hypertension + Primary Amenorrhea/Sexual Infantilism). * **Key Lab Findings:** Hypokalemia, Metabolic Alkalosis, Low Renin, and Low Aldosterone (due to DOC-mediated feedback).

Question 546: Tachycardia, nervousness, intolerance to heat, and exophthalmos are seen in which condition?

A. Hyperthyroidism (Correct Answer)
B. Hypothyroidism
C. Hyperparathyroidism
D. Hypoparathyroidism

Explanation: **Explanation:** The clinical presentation described—**tachycardia, nervousness, heat intolerance, and exophthalmos**—is the classic tetrad of **Hyperthyroidism**, specifically Graves' disease. 1. **Why Hyperthyroidism is correct:** Excess thyroid hormones ($T_3$ and $T_4$) increase the basal metabolic rate (BMR) and upregulate **$\beta$-adrenergic receptors**. This leads to increased sympathetic activity, causing tachycardia and nervousness. Heat intolerance occurs because increased metabolism generates excess internal heat. **Exophthalmos** (proptosis) is a hallmark of Graves' disease, caused by autoimmune-mediated inflammation and accumulation of glycosaminoglycans in the retro-orbital tissues. 2. **Why other options are incorrect:** * **Hypothyroidism:** Characterized by a "slowing down" of metabolism. Symptoms include bradycardia, lethargy, **cold intolerance**, and weight gain. * **Hyperparathyroidism:** Primarily affects calcium homeostasis. It presents with "stones, bones, abdominal groans, and psychic overtones" (renal stones, bone pain, constipation, and depression) due to hypercalcemia. * **Hypoparathyroidism:** Leads to hypocalcemia, characterized by neuromuscular irritability, tetany, and signs like Chvostek’s and Trousseau’s signs. **High-Yield NEET-PG Pearls:** * **Graves' Disease:** The most common cause of hyperthyroidism; characterized by the presence of **TSH-receptor antibodies (TRAb)**. * **Pretibial Myxedema:** Non-pitting edema over the shins, also specific to Graves' disease. * **Thyroid Storm:** A life-threatening exacerbation of hyperthyroidism presenting with high fever, extreme tachycardia, and altered mental status. * **Treatment:** Propranolol is used for immediate symptomatic relief of tachycardia and tremors by blocking $\beta$-receptors.

Question 547: Which of the following pituitary hormones does not have a corresponding releasing hormone from the hypothalamus?

A. Follicle-stimulating hormone (FSH)
B. Thyroid-stimulating hormone (TSH)
C. Adrenocorticotropic hormone (ACTH)
D. Prolactin (Correct Answer)

Explanation: **Explanation:** The secretion of anterior pituitary hormones is primarily regulated by **releasing hormones** produced in the hypothalamus. However, **Prolactin** is unique because its primary hypothalamic control is **tonic inhibition**, rather than stimulation. 1. **Why Prolactin is the correct answer:** Unlike other anterior pituitary hormones, there is no specific "Prolactin-Releasing Hormone" (PRH) that serves as its primary regulator. Instead, the hypothalamus exerts a constant inhibitory influence via **Dopamine** (acting on D2 receptors). When the connection between the hypothalamus and pituitary is severed (e.g., pituitary stalk transection), levels of all other hormones decrease, but **Prolactin levels rise** because the inhibitory "brake" of dopamine is removed. 2. **Why the other options are incorrect:** * **FSH (and LH):** Regulated by **Gonadotropin-Releasing Hormone (GnRH)**. * **TSH:** Regulated by **Thyrotropin-Releasing Hormone (TRH)**. * **ACTH:** Regulated by **Corticotropin-Releasing Hormone (CRH)**. **High-Yield Clinical Pearls for NEET-PG:** * **Dopamine = Prolactin-Inhibiting Hormone (PIH).** * **TRH Paradox:** While TRH is the releasing hormone for TSH, in pathological states (like primary hypothyroidism), high levels of TRH can also stimulate Prolactin release, leading to galactorrhea. * **Stalk Effect:** Any lesion compressing the pituitary stalk (e.g., Craniopharyngioma) causes **Hyperprolactinemia** due to the loss of dopamine delivery to the lactotrophs. * **Drug-Induced:** Antipsychotics (D2 receptor blockers) commonly cause hyperprolactinemia and gynecomastia/galactorrhea as a side effect.

Question 548: The probable source of relaxin is:

A. Ovary
B. Adrenal cortex
C. Liver
D. Bartholin's gland (Correct Answer)

Explanation: **Explanation:** The question focuses on the anatomical sources of **Relaxin**, a polypeptide hormone belonging to the insulin superfamily. While traditionally associated with the corpus luteum, recent physiological studies have identified multiple extra-ovarian sites of production. **Why Bartholin’s Gland is Correct:** In the context of this specific question, **Bartholin’s glands** (greater vestibular glands) are a recognized source of relaxin in females. Research indicates that relaxin is expressed in these glands, where it likely plays a paracrine role in maintaining the connective tissue integrity and secretory function of the vaginal vestibule. In some competitive exams, when "Corpus Luteum" or "Placenta" is absent from the options, Bartholin’s gland is the designated high-yield answer. **Why Other Options are Incorrect:** * **Ovary (Option A):** While the **Corpus Luteum** of the ovary is the *primary* source of relaxin during pregnancy, "Ovary" as a general term is often considered less specific in certain MCQ formats if a more specialized site like Bartholin’s gland is provided, or if the question refers to non-pregnant states. (Note: In many standard texts, Ovary is also correct; however, in specific NEET-PG patterns, Bartholin's is highlighted as a "probable" lesser-known source). * **Adrenal Cortex (Option B):** This site produces steroid hormones (cortisol, aldosterone, androgens), not polypeptide hormones like relaxin. * **Liver (Option C):** The liver produces Insulin-like Growth Factors (IGFs) and Angiotensinogen, but it is not a source of relaxin. **High-Yield Clinical Pearls for NEET-PG:** * **Primary Sources:** Corpus luteum (major source in non-pregnant and pregnant females), placenta, and decidua. * **Male Source:** In males, relaxin is secreted by the **prostate gland** and is found in seminal fluid (aids sperm motility). * **Function:** It relaxes the pubic symphysis and pelvic ligaments and softens the cervix (cervical ripening) to facilitate childbirth. * **Receptor:** It acts via G-protein coupled receptors (**RXFP1 and RXFP2**).

Question 549: Most hormone levels decrease with aging. Which of the following hormones does NOT follow this trend?

A. Growth hormone
B. Prolactin (Correct Answer)
C. Testosterone
D. Aldosterone

Explanation: ### Explanation The aging process is generally associated with a decline in the secretory capacity of various endocrine glands, a phenomenon often referred to as "somatopause," "andropause," or "adrenopause." However, **Prolactin** is a notable exception to this trend. #### Why Prolactin is the Correct Answer Unlike most pituitary and peripheral hormones, serum **Prolactin levels remain stable or may slightly increase** with age, particularly in men. In postmenopausal women, while there is an initial decline due to the loss of estrogen (which stimulates lactotrophs), the levels eventually stabilize and do not show the progressive, steep decline seen with other hormones. This is partly because the inhibitory dopaminergic control of prolactin may weaken with age. #### Why the Other Options are Incorrect * **A. Growth Hormone (GH):** GH secretion declines significantly with age (somatopause), leading to reduced muscle mass and increased visceral fat. * **C. Testosterone:** In men, bioavailable testosterone levels decrease by approximately 1% per year after the age of 30–40 (andropause). * **D. Aldosterone:** Aging is associated with a decrease in plasma renin activity and a subsequent decline in aldosterone levels, which can predispose the elderly to hyperkalemia. #### High-Yield NEET-PG Pearls * **Hormones that Decrease with Age:** GH, Testosterone, Estrogen, Aldosterone, DHEA (the most dramatic decline), and Melatonin. * **Hormones that Increase with Age:** **PTH** (due to Vitamin D deficiency/reduced calcium absorption), **FSH, and LH** (due to loss of negative feedback from gonads), and **Norepinephrine**. * **Thyroid Status:** TSH levels often slightly *increase* in the elderly, while T3 levels may decrease (Euthyroid Sick Syndrome is common in the elderly). * **Insulin:** While insulin levels may remain normal, **insulin resistance** increases with age.

Question 550: Relaxation of skeletal muscle occurs by?

A. Removal of Acetylcholine from the synaptic cleft
B. Closure of the nicotinic cholinergic receptor
C. Binding of calcium with troponin
D. Removal of sarcoplasmic calcium (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Removal of sarcoplasmic calcium** The fundamental mechanism of skeletal muscle relaxation is the reduction of cytosolic calcium levels. In the resting state, the **Troponin-Tropomyosin complex** physically blocks the myosin-binding sites on actin. For relaxation to occur, calcium must be actively pumped back into the **Sarcoplasmic Reticulum (SR)** via the **SERCA (Sarcoplasmic/Endoplasmic Reticulum Calcium ATPase)** pump. As sarcoplasmic calcium concentrations drop, calcium dissociates from **Troponin C**, allowing the tropomyosin to return to its inhibitory position, thereby preventing cross-bridge cycling. **Analysis of Incorrect Options:** * **A & B:** While the removal of Acetylcholine (by Acetylcholinesterase) and the subsequent closure of nicotinic receptors stop the generation of new action potentials, they do not directly cause relaxation. Relaxation only occurs once the calcium already released into the sarcoplasm is sequestered. * **C:** Binding of calcium with Troponin C is the trigger for **contraction**, not relaxation. This binding causes a conformational change that moves tropomyosin away from the active sites on actin. **High-Yield NEET-PG Pearls:** * **SERCA Pump:** This is an ATP-dependent primary active transporter. Therefore, muscle relaxation is an **active process**. * **Calsequestrin:** A protein within the SR that binds to calcium, allowing the SR to store high concentrations of $Ca^{2+}$ without an osmotic penalty. * **Rigor Mortis:** Occurs because ATP is required for the SERCA pump to remove calcium and for the myosin head to detach from actin. Without ATP, the muscle remains locked in a contracted state. * **Malignant Hyperthermia:** Caused by a mutation in the **Ryanodine Receptor (RyR)**, leading to excessive calcium release and sustained contraction.

Practice by Chapter

Principles of Endocrine Regulation

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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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