Endocrinology Practice Questions

Q: Which of the following inhibits aldosterone synthesis in the adrenal cortex?

Atrial natriuretic peptide (ANP). ### Explanation **Correct Answer: B. Atrial natriuretic peptide (ANP)** **Mechanism:** Aldosterone synthesis occurs in the **Zona Glomerulosa** of the adrenal cortex. Atrial Natriuretic Peptide (ANP) is the primary physiological inhibitor of aldosterone. It acts via two main pathways: 1. **Direct Inhibition:** ANP binds to its receptors (NPR-A) in the adrenal cortex, increasing cGMP, which inhibits the synthesis of aldosterone. 2. **Indirect Inhibition:** ANP inhibits the release of **Renin** from the juxtaglomerular cells, thereby decreasing the production of Angiotensin II, the primary stimulator of aldosterone. **Analysis of Incorrect Options:** * **A. ACTH:** While primarily controlling cortisol, ACTH has a **permissive effect** on aldosterone. It stimulates the initial step of steroidogenesis (cholesterol to pregnenolone). High levels of ACTH can acutely increase aldosterone secretion. * **C. Angiotensin I:** This is a relatively inactive precursor. It must be converted to Angiotensin II by ACE (Angiotensin-Converting Enzyme) to exert significant physiological effects. * **D. Angiotensin II:** This is the **most potent stimulator** of aldosterone synthesis. It acts via the $G_q$ protein-coupled receptor to increase intracellular calcium and activate Protein Kinase C, stimulating aldosterone synthase. **High-Yield Clinical Pearls for NEET-PG:** * **Primary Stimulators of Aldosterone:** 1. Hyperkalemia (Direct effect), 2. Angiotensin II (via RAAS), 3. ACTH (Permissive). * **Primary Inhibitors:** 1. ANP/BNP, 2. Hypokalemia, 3. Dopamine. * **Rate-limiting step:** The conversion of corticosterone to aldosterone by the enzyme **Aldosterone Synthase**, which is unique to the Zona Glomerulosa. * **Conn’s Syndrome:** Primary hyperaldosteronism characterized by hypertension, hypokalemia, and metabolic alkalosis with low plasma renin levels.

Q: Which of the following statements regarding oxytocin is true?

All of the above. **Explanation:** Oxytocin is a peptide hormone synthesized in the **paraventricular nucleus** of the hypothalamus and released by the **posterior pituitary**. It plays a pivotal role in reproduction and fluid balance. * **Option A (Milk Ejection Reflex):** Oxytocin causes the contraction of **myoepithelial cells** surrounding the mammary alveoli. This forces milk into the ducts, a process known as "milk let-down" or milk ejection. (Note: Prolactin handles milk *production*, while Oxytocin handles *ejection*). * **Option B (Uterine Contraction):** During labor, stretching of the cervix triggers oxytocin release (Ferguson Reflex). It acts on the G-protein coupled receptors of the uterine myometrium to increase intracellular calcium, causing powerful contractions necessary for parturition. * **Option C (Water Retention):** Oxytocin is structurally very similar to **Vasopressin (ADH)**—both are nonapeptides differing by only two amino acids. At high pharmacological doses (e.g., during prolonged labor induction), oxytocin can cross-react with V2 receptors in the renal tubules, leading to water intoxication and hyponatremia. **Clinical Pearls for NEET-PG:** 1. **Ferguson Reflex:** A positive feedback loop where cervical stretch stimulates oxytocin release, further increasing contractions. 2. **Postpartum Hemorrhage (PPH):** Oxytocin is the first-line drug for PPH because it causes the uterus to contract, compressing bleeding vessels. 3. **Synthesis vs. Storage:** Remember, oxytocin is *synthesized* in the hypothalamus but *stored and released* from the posterior pituitary (Neurohypophysis).

Q: Receptors for thyroid hormone are located in which cellular compartment?

Nucleus. **Explanation:** Thyroid hormones ($T_3$ and $T_4$) are unique among amino acid-derived hormones because they act similarly to steroid hormones. Due to their lipophilic nature, they cross the cell membrane via carrier-mediated transport. **Why the Nucleus is Correct:** The thyroid hormone receptors (TR) are **nuclear receptors** that function as hormone-activated transcription factors. Specifically, $T_3$ binds to these receptors which are already bound to the **Thyroid Response Elements (TRE)** on the DNA. This binding typically triggers the dissociation of co-repressors and the recruitment of co-activators, leading to the transcription of specific mRNA and subsequent protein synthesis (e.g., $Na^+$-$K^+$ ATPase, beta-receptors). **Analysis of Incorrect Options:** * **Cell membrane:** Receptors for peptide hormones (e.g., Insulin, PTH) and catecholamines are located here. While some non-genomic thyroid actions occur at the membrane, the primary physiological receptors are nuclear. * **Cytoplasm:** Receptors for steroid hormones like **Glucocorticoids** are primarily located in the cytoplasm and then translocate to the nucleus. Thyroid receptors are already present in the nucleus. * **DNA strands:** While the receptors are *bound* to DNA, the receptor itself is a protein located within the **Nucleus**. "DNA strands" refers to the genetic material, not the receptor protein. **High-Yield NEET-PG Pearls:** * **Affinity:** $T_3$ has a much higher affinity (approx. 10 times) for the nuclear receptor than $T_4$; hence $T_3$ is the more active form. * **Retinoid X Receptor (RXR):** Thyroid receptors often form a **heterodimer** with RXR at the TRE to initiate transcription. * **Mitochondria:** Thyroid hormones also bind to receptors in the mitochondria to increase ATP production (the "calorigenic effect").

Q: Thyroid hormones in blood are transported by which of the following?

All of the above. **Explanation:** Thyroid hormones ($T_3$ and $T_4$) are highly lipophilic and virtually insoluble in water. Therefore, more than 99% of circulating thyroid hormones are bound to plasma proteins, while only the "free" fraction is biologically active. The transport involves three primary proteins: 1. **Thyroxine-binding Globulin (TBG):** This is the most important carrier. Although present in the lowest concentration, it has the highest affinity for thyroid hormones and carries approximately 70% of circulating $T_4$ and $T_3$. 2. **Transthyretin (Thyroxine-binding Prealbumin - TBPA):** It has a lower affinity than TBG but a higher affinity than albumin. It carries about 10–15% of $T_4$. 3. **Albumin:** It has the lowest affinity but the highest capacity (due to its high plasma concentration). It carries about 15% of $T_4$ and a significant portion of $T_3$. Since all three proteins listed (Albumin, Globulin, and Prealbumin) participate in the transport of thyroid hormones, **Option D** is the correct answer. **High-Yield Clinical Pearls for NEET-PG:** * **Affinity Order:** TBG > Prealbumin > Albumin. * **Capacity Order:** Albumin > Prealbumin > TBG. * **Half-life:** $T_4$ has a longer half-life (7 days) than $T_3$ (1 day) because it binds more tightly to plasma proteins. * **Clinical Correlation:** Conditions that increase TBG (Pregnancy, Estrogen therapy) increase *total* $T_4$ levels, but *free* $T_4$ remains normal (euthyroid state). Conversely, Liver failure or Nephrotic syndrome decreases TBG levels.

Q: Menopausal hot flushes coincide with which of the following?

LH surge. **Explanation:** The correct answer is **LH surge**. **Underlying Medical Concept:** Menopausal hot flushes (vasomotor symptoms) are caused by the narrowing of the thermoregulatory set-point in the hypothalamus. In menopause, the lack of negative feedback from estrogen leads to an increase in **Gonadotropin-Releasing Hormone (GnRH)** pulses. These GnRH pulses stimulate the pituitary to release **Luteinizing Hormone (LH)**. Clinical studies have shown that each episode of a hot flush coincides precisely with a **pulsatile surge of LH**. It is important to note that while the LH surge is the marker, the LH itself is not the cause; rather, the same hypothalamic signals (GnRH and neurokinin B) that trigger the LH pulse also trigger the thermoregulatory dysfunction in the adjacent preoptic nucleus of the hypothalamus. **Analysis of Incorrect Options:** * **A. FSH secretion:** While FSH levels are chronically elevated in menopause (the most sensitive marker), its secretion does not show the same temporal synchronization with individual hot flush episodes as LH. * **B. Decrease in estrogen:** Low estrogen is the *ultimate cause* of menopause, but hot flushes occur in discrete episodes. Estrogen levels do not drop further during the actual flush. * **C. Increase in progesterone:** Progesterone levels are consistently low in menopause due to the absence of ovulation and the corpus luteum. **High-Yield Facts for NEET-PG:** * **Most sensitive marker for Menopause:** Elevated FSH (>40 mIU/ml). * **KNDy Neurons:** Neurons in the hypothalamus (expressing Kisspeptin, Neurokinin B, and Dynorphin) are now considered the "central thermostat" involved in hot flushes. * **Drug of Choice:** Hormone Replacement Therapy (HRT) is the most effective treatment; **SSRIs/SNRIs** or **Gabapentin** are non-hormonal alternatives. * **Fezolinetant:** A newer NK3 receptor antagonist specifically targeting the mechanism of hot flushes.

Q: Aldosterone secretion is stimulated by which of the following?

Hyperkalemia. **Explanation:** Aldosterone is a mineralocorticoid synthesized in the **Zona Glomerulosa** of the adrenal cortex. Its primary role is to maintain electrolyte balance by promoting sodium reabsorption and potassium excretion in the distal convoluted tubule and collecting ducts of the kidney. **Why Hyperkalemia is Correct:** Potassium concentration is the most potent direct stimulator of aldosterone secretion. An increase in serum $K^+$ levels causes direct depolarization of the adrenal cortical cells, opening voltage-gated calcium channels. The resulting calcium influx activates protein kinase C, leading to the synthesis and release of aldosterone to facilitate $K^+$ excretion. **Analysis of Incorrect Options:** * **Dopamine:** This acts as a tonic **inhibitor** of aldosterone secretion. Drugs that block dopamine (like Metoclopramide) can actually lead to a mild increase in aldosterone levels. * **Somatostatin:** This is a universal inhibitory hormone that suppresses various endocrine secretions, including growth hormone and insulin; it also has an **inhibitory** effect on aldosterone. * **Atrial Natriuretic Peptide (ANP):** Released in response to atrial stretch (volume overload), ANP **inhibits** aldosterone secretion to promote natriuresis (sodium loss) and lower blood pressure. **High-Yield Clinical Pearls for NEET-PG:** * **Primary Stimulators:** The two most important stimulators of aldosterone are **Hyperkalemia** and **Angiotensin II** (via the Renin-Angiotensin-Aldosterone System). * **ACTH Role:** ACTH is necessary for aldosterone secretion (permissive action) but is not a primary regulator of its day-to-day levels. * **Conn’s Syndrome:** Primary hyperaldosteronism typically presents with the triad of Hypertension, Hypokalemia, and Metabolic Alkalosis.

Q: What is the greatest stimulator for ADH secretion?

Hyperosmolarity. **Explanation:** **1. Why Hyperosmolarity is Correct:** Antidiuretic Hormone (ADH), or Vasopressin, is primarily regulated by two mechanisms: osmotic and hemodynamic. **Hyperosmolarity** is the **most sensitive** and potent stimulator. Osmoreceptors in the hypothalamus (OVLT and SFO) can detect changes in plasma osmolarity as small as **1%**. When osmolarity rises above the threshold (approx. 280–285 mOsm/L), ADH is released to promote water reabsorption in the renal collecting ducts via V2 receptors. **2. Why Other Options are Incorrect:** * **Hypovolemia (D) and Hypotension (C):** These are potent stimulators but are **less sensitive** than osmolarity. A significant decrease in blood volume or pressure (usually **>10%**) is required to trigger ADH release via baroreceptors. While hypovolemia can override osmolarity in extreme cases (e.g., severe hemorrhage), hyperosmolarity remains the "greatest" (most precise and frequent) physiological stimulator. * **Hyponatremia (B):** This actually **inhibits** ADH secretion. Low sodium levels (hypo-osmolarity) signal the body to excrete excess water to restore osmotic balance. **3. High-Yield Clinical Pearls for NEET-PG:** * **Sensitivity vs. Potency:** Osmolarity is the most *sensitive* trigger (1% change), while Hypovolemia/Hypotension is a more *potent* trigger during emergencies (can cause ADH levels to rise 10x higher than osmotic triggers). * **V1 vs. V2 Receptors:** V1 receptors cause vasoconstriction (at high concentrations), while V2 receptors (via Aquaporin-2) mediate the antidiuretic effect. * **SIADH:** Characterized by excessive ADH despite hyponatremia and low plasma osmolarity. * **Diabetes Insipidus:** Central (lack of ADH) or Nephrogenic (resistance to ADH) leads to polyuria and hyperosmolarity.

Q: Which of the following exerts the maximum effect on parathyroid hormone secretion?

Plasma calcium. **Explanation:** The primary regulator of **Parathyroid Hormone (PTH)** secretion is the ionized **plasma calcium** concentration. This is a classic example of a direct negative feedback loop. The parathyroid glands contain G-protein-coupled receptors called **Calcium-Sensing Receptors (CaSR)**. When plasma calcium levels fall (hypocalcemia), these receptors are less stimulated, leading to an immediate increase in PTH secretion to restore calcium homeostasis via bone resorption and renal reabsorption. **Why other options are incorrect:** * **Plasma Phosphate:** While hyperphosphatemia stimulates PTH secretion, it does so primarily **indirectly** by binding to ionized calcium (forming calcium phosphate salts) and by inhibiting the activation of Vitamin D. It is not as potent or direct a trigger as calcium itself. * **1,25-dihydroxycholecalciferol (Calcitriol):** This is the active form of Vitamin D. It exerts a negative feedback effect on the parathyroid gland to *decrease* PTH synthesis, but it acts on a longer timescale (genomic effect) compared to the rapid response triggered by plasma calcium. * **Calcitonin:** Produced by the parafollicular C-cells of the thyroid, calcitonin functions to lower plasma calcium. While it opposes the action of PTH on bone, it does not directly regulate the secretion of PTH from the parathyroid glands. **NEET-PG High-Yield Pearls:** * **Magnesium's Role:** Low magnesium stimulates PTH, but **severe hypomagnesemia** actually inhibits PTH secretion and action, leading to refractory hypocalcemia. * **PTH Action:** It increases calcium and decreases phosphate (phosphaturic effect) by acting on the proximal and distal tubules of the kidney. * **CaSR Mutation:** A mutation in the CaSR can lead to **Familial Hypocalciuric Hypercalcemia (FHH)**.

Question 1

Which of the following is NOT true about prolactin?

Accepted Answer

Hyperprolactinemia causes a reflex increase in central dopamine levels, which causes menorrhagia.

Answer

Primary hypothyroidism is associated with mild elevation in serum prolactin.

Answer

Normal prolactin level is less than 20 ng/mL in non-pregnant women.

Answer

Dopamine agonists decrease prolactin secretion.

Question 2

Which of the following inhibits aldosterone synthesis in the adrenal cortex?

Accepted Answer

Atrial natriuretic peptide (ANP)

Answer

Adrenocorticotropin (ACTH)

Answer

Angiotensin I

Answer

Angiotensin II

Question 3

Which of the following statements regarding oxytocin is true?

Accepted Answer

All of the above

Answer

Acts on myoepithelial cells of the breast

Answer

Causes contraction of the uterus during labor

Answer

May cause water retention

Question 4

Receptors for thyroid hormone are located in which cellular compartment?

Accepted Answer

Nucleus

Answer

Cell membrane

Answer

Cytoplasm

Answer

DNA strands

Question 5

Thyroid hormones in blood are transported by which of the following?

Accepted Answer

All of the above

Answer

Albumin

Answer

Globulin

Answer

Prealbumin

Question 6

Menopausal hot flushes coincide with which of the following?

Accepted Answer

LH surge

Answer

FSH secretion

Answer

Decrease in estrogen

Answer

Increase in progesterone

Question 7

Aldosterone secretion is stimulated by which of the following?

Accepted Answer

Hyperkalemia

Answer

Dopamine

Answer

Somatostatin

Answer

Atrial Natriuretic Peptide (ANP)

Question 8

What is the greatest stimulator for ADH secretion?

Accepted Answer

Hyperosmolarity

Answer

Hyponatremia

Answer

Hypotension

Answer

Hypovolemia

Question 9

Which of the following exerts the maximum effect on parathyroid hormone secretion?

Accepted Answer

Plasma calcium

Answer

Calcitonin

Answer

Plasma phosphate

Answer

1,25-dihydroxycholecalciferol

Question 10

How is immune rejection in the fetus prevented?

Accepted Answer

HCG

Answer

HPL

Answer

Oestrogen

Answer

Progesterone

Endocrinology — MCQs

Endocrinology — MCQs

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