Endocrinology — MCQs

Endocrinology Indian Medical PG Practice Questions and MCQs

Question 521: Which hormone receptor has 4 subunits and 2 units for tyrosine kinase receptor binding?

A. Insulin (Correct Answer)
B. Glucagon
C. T3
D. ADH

Explanation: ### Explanation The correct answer is **Insulin**. **1. Why Insulin is Correct:** The insulin receptor is a classic example of a **Receptor Tyrosine Kinase (RTK)**. It is a heterotetramer consisting of **four subunits**: * **Two alpha ($\alpha$) subunits:** Located entirely extracellularly, these contain the binding sites for insulin. * **Two beta ($\beta$) subunits:** These are transmembrane proteins. The intracellular portion of each $\beta$ subunit possesses **intrinsic tyrosine kinase activity**. When insulin binds to the $\alpha$ subunits, it triggers autophosphorylation of the tyrosine residues on the $\beta$ subunits, initiating the intracellular signaling cascade (PI3K and MAPK pathways). **2. Why Other Options are Incorrect:** * **Glucagon:** Acts via a **G-Protein Coupled Receptor (GPCR)** linked to the Adenylate Cyclase-cAMP pathway (Gs protein). * **T3 (Triiodothyronine):** Being a lipid-soluble thyroid hormone, it binds to **Nuclear Receptors** (specifically TR$\alpha$ and TR$\beta$) which act as ligand-regulated transcription factors. * **ADH (Vasopressin):** Acts via GPCRs. The **V1 receptor** (vasoconstriction) uses the $G_q$ (IP3/DAG) pathway, while the **V2 receptor** (renal water reabsorption) uses the $G_s$ (cAMP) pathway. **3. High-Yield Clinical Pearls for NEET-PG:** * **IGF-1 (Insulin-like Growth Factor 1)** also uses a similar tetrameric tyrosine kinase receptor. * **Downregulation:** High chronic levels of insulin lead to a decrease in the number of receptors (internalization), contributing to insulin resistance in Type 2 Diabetes. * **GLUT-4:** The primary effect of insulin receptor activation in muscle and adipose tissue is the translocation of GLUT-4 transporters to the cell membrane.

Question 522: Jansen disease is characterized by a defect in which of the following receptors?

A. Parathyroid hormone (PTH) receptor (Correct Answer)
B. Growth hormone (GH) receptor
C. Antidiuretic hormone (ADH) receptor
D. Thyroid hormone receptor

Explanation: **Explanation:** **Jansen’s Metaphyseal Chondrodysplasia** is a rare autosomal dominant disorder caused by an **activating mutation** (gain-of-function) in the **PTH/PTHrP receptor (PTH1R)**. This receptor is a G-protein coupled receptor (GPCR) found primarily in the bone and kidneys. In Jansen disease, the receptor is constitutively active, meaning it signals continuously even in the absence of the hormone. This leads to constant stimulation of bone resorption and renal calcium reabsorption, resulting in the clinical triad of **severe hypercalcemia, hypophosphatemia, and short-limbed dwarfism** (due to abnormal growth plate development). Notably, because the receptor is "always on," endogenous levels of PTH and PTHrP are typically undetectable. **Analysis of Incorrect Options:** * **B. Growth Hormone (GH) receptor:** Defects here lead to **Laron Syndrome** (GH insensitivity), characterized by short stature and high GH levels but low IGF-1. * **C. Antidiuretic Hormone (ADH) receptor:** Mutations in the V2 receptor cause **Nephrogenic Diabetes Insipidus**, leading to polyuria and polydipsia. * **D. Thyroid Hormone receptor:** Mutations here result in **Resistance to Thyroid Hormone (RTH)**, where patients have high T3/T4 levels but may appear hypothyroid or euthyroid. **Clinical Pearls for NEET-PG:** * **PTH1R** is the common receptor for both PTH and PTH-related protein (PTHrP). * **Blomstrand Chondrodysplasia** is the opposite of Jansen’s; it is caused by an **inactivating mutation** (loss-of-function) of the PTH1R, leading to lethal skeletal dysplasia. * **Pseudohypoparathyroidism (Albright’s Hereditary Osteodystrophy)** involves a defect in the **Gsα protein** downstream of the PTH receptor, not the receptor itself.

Question 523: A patient sustains a pituitary stalk transection due to an accident. Which of the following physiological responses will NOT occur?

A. Diabetes mellitus (Correct Answer)
B. Diabetes insipidus
C. Hyperprolactinemia
D. Hypothyroidism

Explanation: **Explanation:** Pituitary stalk transection (infundibular injury) disrupts the connection between the hypothalamus and the pituitary gland. This results in the loss of hypothalamic releasing hormones and the interruption of the hypothalamic-hypophyseal portal system. **Why Diabetes Mellitus is the correct answer:** Diabetes Mellitus is a metabolic disorder characterized by hyperglycemia due to insulin deficiency or resistance. It is related to the **pancreas**, not the pituitary gland. In fact, pituitary stalk transection leads to a loss of Growth Hormone (GH) and ACTH (cortisol), both of which are "diabetogenic" (insulin-antagonistic) hormones. Therefore, stalk injury would more likely lead to **increased insulin sensitivity** or hypoglycemia, rather than Diabetes Mellitus. **Analysis of Incorrect Options:** * **Diabetes Insipidus (B):** The stalk contains the axons of the supraoptic and paraventricular nuclei. Transection prevents ADH (Vasopressin) from reaching the posterior pituitary for release, leading to central diabetes insipidus. * **Hyperprolactinemia (C):** Prolactin is the only anterior pituitary hormone under tonic **inhibition** by hypothalamic dopamine. Stalk transection removes this "dopamine brake," causing prolactin levels to rise. * **Hypothyroidism (D):** Transection prevents Hypothalamic TRH from reaching the anterior pituitary, leading to secondary hypothyroidism (low TSH and T4). **NEET-PG High-Yield Pearls:** 1. **The "Rule of Prolactin":** In any hypothalamic-pituitary destructive lesion, all anterior pituitary hormones decrease **except Prolactin**, which increases. 2. **Panhypopituitarism:** Stalk injury typically causes deficiency of GH, LH/FSH, TSH, and ACTH. 3. **Houssay Phenomenon:** This refers to the spontaneous improvement of hyperglycemia in a diabetic patient who subsequently develops pituitary insufficiency (due to loss of GH and Cortisol).

Question 524: Which one of the following makes epiphyseal cartilage matrix fail to calcify?

A. Thyroxine
B. Hypervitaminosis A
C. Hypovitaminosis D (Correct Answer)
D. Hydrocortisone

Explanation: **Explanation:** **Why Hypovitaminosis D is correct:** Vitamin D (Calcitriol) is essential for the mineralization of the bone matrix. Its primary role is to maintain adequate serum levels of Calcium and Phosphate by increasing intestinal absorption. In **Hypovitaminosis D**, there is a deficiency of these minerals in the extracellular fluid. Consequently, while the chondrocytes continue to proliferate and create the osteoid (organic matrix) at the epiphyseal plate, this matrix **fails to calcify**. This leads to the accumulation of unmineralized osteoid, resulting in the clinical condition known as **Rickets** in children and **Osteomalacia** in adults. **Why the other options are incorrect:** * **Thyroxine (A):** Thyroid hormones are essential for linear growth and skeletal maturation. Deficiency leads to delayed epiphyseal closure and stunting (Cretinism), but it does not specifically cause a failure of matrix calcification. * **Hypervitaminosis A (B):** Excessive Vitamin A accelerates the resorption of cartilage and bone, leading to premature closure of the epiphyses and thinning of the bone cortex, rather than a failure of calcification. * **Hydrocortisone (D):** Glucocorticoids generally inhibit bone formation by decreasing osteoblast activity and reducing intestinal calcium absorption, but they do not specifically target the calcification mechanism of the epiphyseal cartilage matrix in the same manner as Vitamin D deficiency. **High-Yield NEET-PG Pearls:** * **Rickets Hallmark:** Failure of osteoid mineralization + expansion of the epiphyseal plate (widening/cupping/fraying on X-ray). * **Vitamin D Pathway:** 7-dehydrocholesterol → Cholecalciferol (Skin) → 25-OH Vit D (Liver) → 1,25-(OH)₂ Vit D (Kidney via 1-alpha-hydroxylase). * **Alkaline Phosphatase (ALP):** Characteristically **elevated** in Vitamin D deficiency as osteoblasts attempt to compensate for the lack of mineralization.

Question 525: Hyperkalemia stimulates the secretion of which hormone?

A. Antidiuretic hormone (ADH)
B. Secretin
C. Aldosterone (Correct Answer)
D. Parathormone

Explanation: **Explanation:** **Aldosterone** is the primary mineralocorticoid secreted by the *zona glomerulosa* of the adrenal cortex. Its secretion is regulated by two main stimuli: the **Renin-Angiotensin-Aldosterone System (RAAS)** and **plasma potassium levels**. When plasma potassium levels rise (Hyperkalemia), it directly depolarizes the membranes of the zona glomerulosa cells. This opens voltage-gated calcium channels, leading to an influx of calcium, which triggers the synthesis and release of aldosterone. Once secreted, aldosterone acts on the **principal cells** of the late distal tubule and collecting duct to increase the activity of Na⁺/K⁺-ATPase pumps and ENaC channels. This results in sodium reabsorption and, crucially, **increased potassium secretion** into the urine, thereby restoring potassium homeostasis. **Why other options are incorrect:** * **Antidiuretic hormone (ADH):** Primarily regulated by plasma osmolarity and blood volume; its main role is water reabsorption via aquaporins. * **Secretin:** A gastrointestinal hormone stimulated by acidic chyme in the duodenum; it promotes bicarbonate secretion from the pancreas. * **Parathormone (PTH):** Regulated by low ionized serum calcium levels; it acts to increase calcium and decrease phosphate levels. **High-Yield NEET-PG Pearls:** * **Direct Stimulus:** Potassium is the most potent direct stimulus for aldosterone; even a 0.1 mEq/L rise can significantly increase secretion. * **Conn’s Syndrome:** Primary hyperaldosteronism typically presents with the triad of hypertension, hypokalemia, and metabolic alkalosis. * **Inverse Relationship:** While hyperkalemia stimulates aldosterone, aldosterone deficiency (as seen in Addison’s disease) leads to life-threatening hyperkalemia.

Question 526: In a patient, if the administration of vasopressin does not increase the osmolality of urine, what does it indicate?

A. Antidiuretic hormone (ADH) deficiency
B. Renal hyposensitivity towards ADH (Correct Answer)
C. Syndrome of Inappropriate Antidiuretic Hormone secretion (SIADH)
D. Psychogenic polydipsia

Explanation: ### Explanation The core of this question lies in differentiating between the types of **Diabetes Insipidus (DI)** using the **Vasopressin (Desmopressin) Challenge Test**. #### 1. Why the Correct Answer is Right In a normal individual or a patient with central DI, the administration of exogenous Vasopressin (ADH) acts on the V2 receptors in the renal collecting ducts, increasing water reabsorption and significantly raising urine osmolality. However, in **Nephrogenic Diabetes Insipidus (Renal hyposensitivity)**, the kidneys are unresponsive to ADH due to genetic mutations (e.g., V2 receptor or Aquaporin-2 defects) or acquired causes (e.g., Lithium toxicity). Therefore, even after giving Vasopressin, the urine remains dilute (low osmolality) because the renal "machinery" cannot respond to the hormone. #### 2. Why Other Options are Incorrect * **ADH Deficiency (Central DI):** Here, the posterior pituitary fails to secrete ADH. Since the kidneys are healthy, administering exogenous Vasopressin will result in a **significant increase** (>50%) in urine osmolality. * **SIADH:** This condition involves excessive ADH secretion, leading to concentrated urine and hyponatremia. Administering more Vasopressin is not a diagnostic step and would not explain a failure to concentrate urine. * **Psychogenic Polydipsia:** This is primary excessive water intake. The ADH mechanism is intact. Following water deprivation, these patients can concentrate their urine, and they respond normally to Vasopressin. #### 3. High-Yield Clinical Pearls for NEET-PG * **Water Deprivation Test:** The first step in diagnosing polyuria. If urine osmolality remains low (<300 mOsm/kg), proceed to the Vasopressin test. * **Response Threshold:** A >50% increase in urine osmolality after Desmopressin indicates **Central DI**; a <10% increase indicates **Nephrogenic DI**. * **Drug of Choice:** Desmopressin (DDAVP) is the treatment for Central DI, while Thiazide diuretics or Amiloride (especially for Lithium-induced) are used for Nephrogenic DI.

Question 527: Prolactin secretion is increased by all EXCEPT:

A. Dopamine (Correct Answer)
B. Sleep
C. Pregnancy
D. Stress

Explanation: **Explanation:** The regulation of prolactin 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 portal hypophyseal vessels. It acts on **D2 receptors** located on lactotrophs to inhibit the synthesis and secretion of prolactin. Therefore, dopamine *decreases* prolactin levels, making it the correct "EXCEPT" choice. **2. Why the Other Options are Incorrect:** * **Sleep:** Prolactin secretion is pulsatile and follows a diurnal rhythm. Levels rise significantly during sleep (especially non-REM), peaking in the early morning hours. * **Pregnancy:** High levels of estrogen during pregnancy stimulate the hypertrophy and hyperplasia of lactotrophs, leading to a marked increase in prolactin secretion. * **Stress:** Prolactin is considered a "stress hormone." Physical or emotional stress (including exercise, surgery, or hypoglycemia) triggers a rapid release of prolactin via serotonergic and opioid pathways. **High-Yield Clinical Pearls for NEET-PG:** * **TRH Connection:** Thyrotropin-releasing hormone (TRH) acts as a potent prolactin-releasing factor. This explains why patients with **Primary Hypothyroidism** (high TRH) often present with hyperprolactinemia. * **Drug-Induced Hyperprolactinemia:** Any drug that blocks D2 receptors (e.g., Antipsychotics like Haloperidol, or Prokinetics like Metoclopramide) will cause elevated prolactin levels. * **Stalk Effect:** Any pituitary tumor or trauma that compresses the pituitary stalk prevents dopamine from reaching the lactotrophs, leading to increased prolactin (the only hormone that increases when the stalk is cut).

Question 528: Prolactin secretion is not stimulated by:

A. Levodopa (Correct Answer)
B. Sleep
C. Estrogen
D. Pregnancy

Explanation: **Explanation:** The regulation of prolactin (PRL) is unique among anterior pituitary hormones because it is primarily under **tonic inhibition** by the hypothalamus. The major prolactin-inhibiting factor (PIF) is **Dopamine**, which acts on D2 receptors of the lactotrophs. **Why Levodopa is the Correct Answer:** Levodopa (L-Dopa) is a precursor to dopamine. When administered, it crosses the blood-brain barrier and increases dopamine levels in the brain. Since dopamine inhibits prolactin release, **Levodopa decreases prolactin secretion** rather than stimulating it. Therefore, it is the correct answer as it is an inhibitor. **Analysis of Incorrect Options:** * **Sleep:** Prolactin secretion follows a pulsatile and diurnal rhythm. Levels rise significantly during sleep (especially during non-REM sleep), peaking in the early morning hours. * **Estrogen:** Estrogen stimulates the hypertrophy and hyperplasia of lactotrophs and directly increases prolactin gene expression. This is why prolactin levels are higher in females after puberty. * **Pregnancy:** During pregnancy, high levels of estrogen and progesterone lead to a progressive increase in prolactin levels (up to 10-20 fold) to prepare the mammary glands for lactation. **High-Yield Clinical Pearls for NEET-PG:** * **TRH (Thyrotropin-Releasing Hormone):** While primarily for TSH, TRH is a potent stimulator of prolactin. This explains why patients with **Primary Hypothyroidism** often have hyperprolactinemia. * **Dopamine Antagonists:** Drugs like antipsychotics (Haloperidol) and prokinetics (Metoclopramide) block D2 receptors, leading to hyperprolactinemia and galactorrhea. * **Suckling Reflex:** This is the most potent physiological stimulus for prolactin release, mediated by a neuroendocrine reflex that inhibits dopamine release.

Question 529: FSH is secreted by:

A. Ovary
B. Hypothalamus
C. Anterior pituitary (Correct Answer)
D. Posterior pituitary

Explanation: **Explanation:** **Follicle-Stimulating Hormone (FSH)** is a gonadotropin synthesized and secreted by the **gonadotroph cells** of the **Anterior Pituitary (Adenohypophysis)**. Its secretion is regulated by the pulsatile release of Gonadotropin-Releasing Hormone (GnRH) from the hypothalamus. * **Why Option C is correct:** The anterior pituitary produces six major hormones: Growth Hormone (GH), Prolactin (PRL), Thyroid-Stimulating Hormone (TSH), Adrenocorticotropic Hormone (ACTH), Luteinizing Hormone (LH), and FSH. FSH plays a critical role in gametogenesis—stimulating follicular development in females and spermatogenesis (via Sertoli cells) in males. * **Why Option A is incorrect:** The ovaries are the *target organ* for FSH, not the source. In response to FSH, the ovaries produce Estrogen and Inhibin. * **Why Option B is incorrect:** The hypothalamus produces **GnRH**, which acts as a secretagogue for FSH, but it does not secrete FSH itself. * **Why Option D is incorrect:** The posterior pituitary (Neurohypophysis) does not synthesize hormones; it only stores and releases **Oxytocin** and **Antidiuretic Hormone (ADH/Vasopressin)**, which are produced in the hypothalamus. **High-Yield Clinical Pearls for NEET-PG:** * **Structure:** FSH is a glycoprotein consisting of an **alpha (α) subunit** (identical to LH, TSH, and hCG) and a **beta (β) subunit**, which provides biological specificity. * **Feedback:** FSH is specifically inhibited by **Inhibin B**, produced by granulosa cells (females) and Sertoli cells (males). * **Clinical Marker:** Elevated FSH levels (>40 mIU/mL) are the gold-standard biochemical marker for diagnosing **Menopause** or Premature Ovarian Failure.

Question 530: What is the efferent pathway for the milk ejection reflex?

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

Explanation: The **Milk Ejection Reflex** (also known as the "Let-down reflex") is a neuroendocrine reflex initiated by suckling. ### **Explanation of the Correct Answer (B)** When an infant suckles, tactile receptors on the nipple send afferent impulses via the somatic nerves to the **supraoptic and paraventricular nuclei** of the hypothalamus. This triggers the release of **Oxytocin** from the posterior pituitary into the bloodstream. Oxytocin acts as the **efferent hormonal pathway**, traveling to the mammary glands where it causes the contraction of **myoepithelial cells** surrounding the alveoli. This contraction forces milk from the alveoli into the ducts and out through the nipple. ### **Why Other Options are Incorrect** * **A. Prolactin:** While essential for lactation, Prolactin is responsible for the **production and secretion** of milk (milk synthesis) within the alveolar cells, not its ejection. * **C. ACTH:** Adrenocorticotropic hormone stimulates the adrenal cortex to release cortisol. It has no direct role in the acute milk ejection reflex. * **D. Growth Hormone:** GH is involved in the development of mammary tissue (mammogenesis) and general metabolism but does not trigger milk let-down. ### **NEET-PG High-Yield Pearls** * **Afferent Pathway:** Neural (Somatic nerves to Hypothalamus). * **Efferent Pathway:** Humoral/Hormonal (Oxytocin via blood). * **Psychogenic Inhibition:** Stress, fear, or pain can inhibit oxytocin release (via increased sympathetic tone), thereby blocking the milk ejection reflex. * **Conditioned Reflex:** The reflex can be "conditioned"; the mere sight or cry of the baby can trigger oxytocin release and milk let-down. * **Uterine Effect:** Oxytocin released during breastfeeding also causes uterine contractions (involution), helping the uterus return to its pre-pregnancy size.

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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