Endocrinology — MCQs

Endocrinology Indian Medical PG Practice Questions and MCQs

Question 171: Erythropoietin is secreted by which organ?

A. Pituitary
B. Lung
C. Spleen
D. Kidney (Correct Answer)

Explanation: **Explanation:** **Erythropoietin (EPO)** is a glycoprotein hormone that serves as the primary regulator of erythropoiesis (red blood cell production). 1. **Why Kidney is Correct:** In adults, approximately **85–90% of EPO** is synthesized and secreted by the **peritubular interstitial cells (fibroblasts)** in the renal cortex. These cells act as oxygen sensors; when they detect renal hypoxia (low $pO_2$), they trigger the expression of Hypoxia-Inducible Factor (HIF-1), which stimulates EPO production. The EPO then travels to the bone marrow to stimulate the proliferation and differentiation of erythroid progenitor cells. 2. **Why Other Options are Incorrect:** * **Pituitary:** Secretes hormones like GH, TSH, and ACTH, but has no role in EPO production. * **Lung:** While the lungs are involved in oxygen exchange, they do not produce EPO. * **Spleen:** The spleen is involved in the destruction of old RBCs (sequestration) and acts as a blood reservoir, but it does not secrete EPO. **High-Yield Clinical Pearls for NEET-PG:** * **Site of Production:** In the **fetus**, the **liver** is the primary source of EPO. After birth, the site shifts to the **kidneys**. * **Stimulus:** The primary stimulus for EPO release is **hypoxia**, not the number of RBCs themselves. * **Clinical Correlation:** Patients with **Chronic Kidney Disease (CKD)** often develop normocytic normochromic anemia due to the deficiency of EPO. * **Polycythemia:** Conditions like Renal Cell Carcinoma (RCC) or Hepatocellular Carcinoma (HCC) can lead to paraneoplastic secretion of EPO, causing secondary polycythemia.

Question 172: Fertilization occurs at?

A. Cervix
B. Uterus
C. Ampulla (Correct Answer)
D. Ovary

Explanation: Fertilization typically occurs in the **ampulla of the fallopian tube**. The ampulla is the widest and longest portion of the uterine tube, making it the ideal physiological site for the sperm to meet the secondary oocyte. After ovulation, the fimbriae pick up the oocyte and transport it to the ampulla, where it remains viable for approximately 12–24 hours. Spermatozoa, having undergone capacitation in the female reproductive tract, reach this site to initiate the acrosome reaction. **2. Why Other Options are Incorrect:** * **Cervix:** This serves as the entry point for sperm and acts as a reservoir, but its acidic environment and mucus architecture are not conducive to the complex process of fertilization. * **Uterus:** While the uterus is the site for **implantation** (specifically the posterior wall of the fundus), fertilization must occur earlier in the fallopian tube to allow the zygote to undergo initial cleavage divisions into a blastocyst before reaching the uterine cavity. * **Ovary:** The ovary is the site of oogenesis and ovulation. Once the egg is released, it leaves the ovary; fertilization inside the ovary is pathological (primary ovarian pregnancy). **3. NEET-PG High-Yield Pearls:** * **Timing:** Fertilization usually occurs within 12–24 hours after ovulation. Some sperm can survive in the female tract and fertilize the ovum for up to 120 hours, though fertility is highest within 48 hours [1]. * **Capacitation:** This vital process (removal of glycoprotein coat) occurs in the uterus and fallopian tubes, taking about 7 hours; without it, sperm cannot fertilize the egg. * **Ectopic Pregnancy:** The **ampulla** is also the most common site for ectopic pregnancies (approx. 70-80%). * **Sequence:** Zygote → Morula (16-cell stage, enters uterus on day 4) → Blastocyst (implants on day 6).

Question 173: Protein-bound iodine measures the secretory function of the thyroid in all of the following circumstances except?

A. Nephrotic syndrome (Correct Answer)
B. Following hemithyroidectomy
C. During ampicillin therapy
D. Asthmatics on ephedrine

Explanation: **Explanation** The **Protein-Bound Iodine (PBI)** test measures the amount of iodine attached to serum proteins, primarily **Thyroxine-Binding Globulin (TBG)**. Since over 99% of circulating thyroid hormones (T4 and T3) are protein-bound, PBI was historically used as an indirect surrogate for thyroid secretory function. **Why Nephrotic Syndrome is the correct answer:** In **Nephrotic Syndrome**, there is massive proteinuria leading to a significant loss of transport proteins, including TBG, in the urine. This results in a **falsely low PBI level**, even if the thyroid gland's secretory function is perfectly normal (euthyroid). Because the test depends on the concentration of carrier proteins rather than just the hormone produced, it fails to accurately reflect thyroid status in protein-losing states. **Analysis of Incorrect Options:** * **Following Hemithyroidectomy:** The remaining thyroid tissue usually compensates or reflects a true decrease in secretory output; thus, PBI still correlates with the actual hormone production of the remaining gland. * **Ampicillin Therapy:** Ampicillin does not significantly interfere with TBG levels or iodine binding, allowing PBI to remain a valid (though dated) measure of secretion. * **Asthmatics on Ephedrine:** Ephedrine is a sympathomimetic that does not alter TBG concentration or thyroid hormone binding; therefore, PBI remains reflective of thyroid function. **High-Yield Clinical Pearls for NEET-PG:** * **Modern Standard:** PBI is now obsolete, replaced by **Serum TSH** (most sensitive screening test) and **Free T4/T3** assays, which are unaffected by protein fluctuations. * **TBG Variations:** Factors that **increase TBG** (falsely raising PBI/Total T4) include pregnancy and oral contraceptives (estrogen). Factors that **decrease TBG** (falsely lowering PBI/Total T4) include Nephrotic syndrome, liver failure, and androgens. * **Drug Interference:** Phenytoin and Salicylates can displace T4 from TBG, affecting total hormone levels but not the free (active) fraction.

Question 174: Injection of hypertonic saline into which region of the hypothalamus produces intense thirst?

A. Posterior region
B. Paraventricular nucleus
C. Preoptic nucleus (Correct Answer)
D. Supraoptic nucleus

Explanation: **Explanation:** The regulation of thirst and water intake is primarily governed by **osmoreceptors** located in the **Anteroventral Third Ventricle (AV3V) region** of the hypothalamus. This region includes the **Preoptic nucleus** (specifically the median preoptic nucleus) and the organum vasculosum of the lamina terminalis (OVLT). 1. **Why Preoptic Nucleus is Correct:** The neurons in the preoptic area act as osmoreceptors. When hypertonic saline is injected, it increases the osmolarity of the extracellular fluid. This causes water to move out of these osmoreceptor cells by osmosis, causing them to shrink. This physical shrinkage triggers nerve impulses that project to the cerebral cortex to create the conscious sensation of **thirst** and to the supraoptic/paraventricular nuclei to release ADH. 2. **Why other options are incorrect:** * **Posterior region:** This area is primarily involved in thermoregulation (response to cold) and arousal; it is not a primary center for thirst. * **Supraoptic and Paraventricular nuclei:** While these nuclei are the sites of **synthesis** for Antidiuretic Hormone (ADH/Vasopressin) in response to osmotic changes, they are the *effectors* of water conservation rather than the primary sensory trigger for the *sensation* of thirst. **High-Yield Clinical Pearls for NEET-PG:** * **Thirst Center:** Located in the lateral hypothalamus and the preoptic area. * **Osmoreceptors:** Lack a blood-brain barrier, allowing them to sense plasma osmolarity directly. * **Stimuli for Thirst:** Increased plasma osmolarity (most potent), decreased ECF volume (via Angiotensin II), and dry mouth. * **Adipsia:** Damage to the anterior hypothalamus can lead to a total loss of thirst, even in the presence of severe dehydration.

Question 175: Insulin release is inhibited by:

A. Somatostatin (Correct Answer)
B. Glucagon
C. Acetylcholine
D. Amino acids

Explanation: ### Explanation **Correct Answer: A. Somatostatin** **Mechanism of Action:** Insulin secretion from the pancreatic **Beta cells** is regulated by a complex interplay of nutrients, hormones, and neural inputs. **Somatostatin**, secreted by the **Delta ($\delta$) cells** of the pancreas, acts as a potent universal inhibitor. It exerts a paracrine effect on neighboring Beta cells by binding to **SSTR-5 receptors**, which are coupled to inhibitory G-proteins ($G_i$). This leads to a decrease in intracellular cAMP and the inhibition of calcium influx, effectively shutting down insulin release. **Analysis of Incorrect Options:** * **B. Glucagon:** Secreted by Alpha cells, glucagon actually **stimulates** insulin release. This paracrine interaction ensures that as glucagon raises blood glucose, insulin is available to facilitate glucose uptake by peripheral tissues. * **C. Acetylcholine:** This is the primary neurotransmitter of the parasympathetic nervous system (Vagus nerve). It **stimulates** insulin secretion (via $M_3$ receptors) in anticipation of nutrient absorption during the "rest and digest" phase. * **D. Amino Acids:** Arginine and Lysine are potent **secretagogues** for insulin. Their presence signals a fed state, necessitating insulin for protein synthesis and glucose regulation. **High-Yield NEET-PG Pearls:** * **Most potent stimulator:** Glucose is the primary physiological stimulus for insulin. * **Incretin Effect:** Oral glucose causes a greater insulin response than IV glucose due to hormones like **GLP-1 and GIP**. * **Adrenergic Control:** $\alpha_2$-adrenergic stimulation **inhibits** insulin (dominant during stress/exercise), while $\beta_2$-stimulation **increases** it. * **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 176: Albinism is due to:

A. Marked deficiency of melanin pigment (Correct Answer)
B. Thymus disorder
C. Tumour of pineal gland
D. ACTH deficiency

Explanation: **Explanation:** **Albinism** is a group of inherited genetic disorders characterized by a **marked deficiency or total absence of melanin pigment** in the skin, hair, and eyes. The most common cause is a defect in the enzyme **Tyrosinase**, which is essential for converting the amino acid Tyrosine into DOPA and subsequently into Melanin within melanocytes. * **Why Option A is correct:** In albinism, melanocytes are present in normal numbers, but they fail to produce melanin due to enzymatic defects (most commonly Tyrosinase-negative Oculocutaneous Albinism). This lack of pigment leads to photophobia, nystagmus, and an increased risk of skin cancers (Squamous Cell Carcinoma). **Analysis of Incorrect Options:** * **B. Thymus disorder:** The thymus is involved in T-cell maturation and immune surveillance (e.g., Myasthenia Gravis or DiGeorge Syndrome); it has no role in melanogenesis. * **C. Tumour of pineal gland:** Pineal tumors can affect Melatonin secretion (which regulates circadian rhythms) or cause Parinaud syndrome, but they do not cause the systemic loss of melanin seen in albinism. * **D. ACTH deficiency:** ACTH shares a precursor with Melanocyte-Stimulating Hormone (MSH) called **POMC**. While ACTH *excess* (as in Addison’s disease) causes hyperpigmentation, ACTH *deficiency* leads to skin pallor, not the congenital lack of pigment characteristic of albinism. **High-Yield Clinical Pearls for NEET-PG:** * **Vitiligo vs. Albinism:** Vitiligo is an acquired autoimmune destruction of melanocytes (decreased melanocyte count), whereas Albinism is a congenital enzyme defect (normal melanocyte count, decreased melanin). * **Chediak-Higashi Syndrome:** A high-yield related condition featuring partial albinism, giant lysosomal granules, and recurrent infections. * **Waardenburg Syndrome:** Characterized by patchy albinism (white forelock) and sensorineural deafness.

Question 177: If a normal individual receives an insulin injection that lowers plasma glucose to 30 mg/dl, which of the following hormones will not be released?

A. ACTH
B. Epinephrine
C. Growth hormone
D. Aldosterone (Correct Answer)

Explanation: **Explanation:** The physiological response to hypoglycemia (plasma glucose <70 mg/dl) involves the activation of the **Counter-regulatory Hormone System**. This system aims to restore glucose levels through glycogenolysis and gluconeogenesis. **Why Aldosterone is the Correct Answer:** Aldosterone is a mineralocorticoid primarily regulated by the **Renin-Angiotensin-Aldosterone System (RAAS)** in response to low blood pressure or hypovolemia, and by serum **Potassium ($K^+$)** levels. It is not a counter-regulatory hormone. Hypoglycemia does not trigger the RAAS or significant electrolyte shifts that would necessitate aldosterone release. **Why the other options are incorrect:** * **ACTH (Option A):** Hypoglycemia is a potent stressor that activates the Hypothalamic-Pituitary-Adrenal (HPA) axis. The hypothalamus releases CRH, which stimulates the anterior pituitary to release **ACTH**, subsequently increasing **Cortisol**. Cortisol promotes gluconeogenesis. * **Epinephrine (Option B):** This is the "first line" rapid response to hypoglycemia. Low glucose triggers the sympathetic nervous system and the adrenal medulla to release epinephrine, which immediately stimulates hepatic glycogenolysis. * **Growth Hormone (Option C):** GH is released in response to hypoglycemia to decrease peripheral glucose utilization and promote lipolysis, acting as a glucose-sparing agent. **High-Yield NEET-PG Pearls:** 1. **Hierarchy of Response:** The first defense against hypoglycemia is the suppression of endogenous insulin, followed by a rise in **Glucagon** and **Epinephrine**. 2. **Insulin Tolerance Test (ITT):** Inducing hypoglycemia with insulin is the "Gold Standard" test to evaluate the integrity of the HPA axis and Growth Hormone reserve. 3. **Glucagon vs. Epinephrine:** Glucagon is the most important hormone for acute recovery from hypoglycemia in non-diabetics; however, if glucagon is deficient, epinephrine becomes critical.

Question 178: Semen is released by which of the following structures?

A. Epididymis
B. Testes
C. Vas deferens (Correct Answer)
D. Prostate

Explanation: **Explanation:** The process of ejaculation involves the coordinated movement of sperm and glandular secretions into the urethra. The **Vas deferens** is the correct answer because it acts as the primary conduit that propels mature spermatozoa from the epididymis to the ejaculatory ducts via peristaltic contractions during the emission phase of the male sexual act. While semen is a mixture of fluids, the final "release" into the posterior urethra for ejaculation is mediated by the vas deferens. **Analysis of Options:** * **Testes (B):** These are the primary sites of spermatogenesis (production of sperm) and testosterone secretion. They do not release "semen" (the final mixture); they only provide the cellular component (spermatozoa). * **Epididymis (A):** This is the site for sperm maturation and storage. While sperm gain motility here, the epididymis does not release the bulk of the seminal fluid or the final ejaculate. * **Prostate (D):** The prostate gland secretes a thin, milky, alkaline fluid that makes up about 30% of the semen volume. While it contributes to the composition, it is a secretory gland rather than the primary structure that "releases" the combined semen into the tract. **High-Yield NEET-PG Pearls:** * **Composition of Semen:** Seminal vesicles contribute ~60% (fructose, prostaglandins), Prostate ~30% (citrate, calcium, phosphate, profibrinolysin), and Vas deferens ~10% (sperm). * **Emission vs. Ejaculation:** Emission (movement into the urethra) is mediated by **Sympathetic** nerves (L1-L2), while Ejaculation (expulsion from the urethra) is mediated by the **Pudendal nerve** (S2-S4). * **Fructose:** Produced exclusively by seminal vesicles; its presence in semen is a marker for the patency of the ejaculatory ducts.

Question 179: Which of the following receptor genes controls TSH secretion?

A. TR alpha 1
B. TR alpha 2
C. TR beta 1
D. TR beta 2 (Correct Answer)

Explanation: ### Explanation **Correct Option: D (TR beta 2)** The regulation of the Hypothalamic-Pituitary-Thyroid (HPT) axis relies on the negative feedback of thyroid hormones ($T_3$ and $T_4$). This feedback is mediated by specific nuclear **Thyroid Hormone Receptors (TR)**. The **TR beta 2 ($\text{TR}\beta_2$)** isoform is specifically expressed in the **anterior pituitary gland** and the **hypothalamus**. When $T_3$ binds to $\text{TR}\beta_2$ in the pituitary, it inhibits the transcription of the TSH-$\beta$ subunit gene, thereby decreasing TSH secretion. In the hypothalamus, it inhibits the synthesis of Thyrotropin-Releasing Hormone (TRH). Therefore, $\text{TR}\beta_2$ is the primary gatekeeper for the set-point of the thyroid axis. --- ### Analysis of Incorrect Options * **TR alpha 1 ($\text{TR}\alpha_1$):** Predominantly expressed in the **heart** and skeletal muscle. It mediates the chronotropic and inotropic effects of thyroid hormones (e.g., increased heart rate). * **TR alpha 2 ($\text{TR}\alpha_2$):** This is a splice variant that **cannot bind $T_3$**. It acts as an antagonist or a "dominant negative" receptor, potentially inhibiting the action of other TR isoforms. * **TR beta 1 ($\text{TR}\beta_1$):** Widely distributed but found in high concentrations in the **liver and kidneys**. It is primarily responsible for the metabolic effects of thyroid hormones, such as cholesterol lowering and thermogenesis. --- ### High-Yield Clinical Pearls for NEET-PG * **Resistance to Thyroid Hormone (RTH) Syndrome:** Most commonly caused by mutations in the **TR beta gene**. Patients present with elevated $T_3/T_4$ levels but inappropriately normal or high TSH because the pituitary "sensor" ($\text{TR}\beta_2$) is defective. * **Gene Locations:** TR-alpha is encoded on **Chromosome 17**, while TR-beta is encoded on **Chromosome 3**. * **Receptor Type:** Thyroid receptors are **Nuclear Receptors** that function as ligand-activated transcription factors, usually forming heterodimers with the Retinoid X Receptor (RXR).

Question 180: Thyroid hormones belong to which class of hormone?

A. Steroids
B. Proteins
C. Polypeptides
D. Amino acid derivatives (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Amino acid derivatives** Thyroid hormones (T3 and T4) are synthesized from the amino acid **Tyrosine**. Specifically, they are formed by the iodination of tyrosine residues within the thyroglobulin molecule. Despite being amino acid derivatives, they are unique because they are **lipophilic** (lipid-soluble), allowing them to cross cell membranes and bind to **nuclear receptors**, similar to steroid hormones. **Analysis of Incorrect Options:** * **A. Steroids:** These are derived from **cholesterol**. Examples include cortisol, aldosterone, estrogen, and testosterone. While thyroid hormones share a similar mechanism of action (nuclear binding), their chemical precursor is an amino acid, not a lipid. * **B & C. Proteins and Polypeptides:** These consist of chains of amino acids linked by peptide bonds. Examples include Insulin, PTH, and Anterior Pituitary hormones. Unlike thyroid hormones, these are water-soluble, circulate freely in the blood, and bind to **cell surface receptors**. **High-Yield Clinical Pearls for NEET-PG:** * **The Tyrosine Connection:** Tyrosine is the precursor for three major groups: Thyroid hormones, Catecholamines (Epinephrine, Norepinephrine, Dopamine), and Melanin. * **Storage Exception:** Thyroid hormone is the only endocrine hormone stored extracellularly (within the **colloid** of the thyroid follicle) in large quantities—enough to last for 2–3 months. * **Transport:** Because they are lipophilic, >99% of T3/T4 circulate bound to plasma proteins, primarily **Thyroxine-binding globulin (TBG)**. * **Potency:** T3 is more potent and has a faster onset of action, while T4 is produced in higher quantities and has a longer half-life.

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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