Endocrine Pharmacology Practice Questions

Q: Allopurinol specifically inhibits which enzyme?

Xanthine oxidase. ### Explanation **Correct Option: A. Xanthine Oxidase** Allopurinol is a structural analogue of hypoxanthine. It acts as a **competitive inhibitor** of the enzyme **Xanthine Oxidase (XO)**. In the body, allopurinol is metabolized by XO into **alloxanthine (oxypurinol)**, which then acts as a potent non-competitive inhibitor of the same enzyme (suicide inhibition). By inhibiting XO, allopurinol prevents the conversion of hypoxanthine to xanthine and xanthine to **uric acid**, thereby lowering serum urate levels. This makes it the drug of choice for the chronic management of gout and hyperuricemia. **Incorrect Options:** * **B. Arginase:** This enzyme is part of the Urea Cycle, responsible for converting Arginine into Urea and Ornithine. It is not involved in purine metabolism. * **C. Carbamoyl transferase:** Specifically, Ornithine Transcarbamoylase (OTC) is an enzyme of the Urea Cycle. Deficiencies here lead to hyperammonemia, not hyperuricemia. * **D. Urease:** This enzyme is produced by bacteria (like *H. pylori*) to neutralize gastric acid by converting urea into ammonia and CO₂. It is not a human metabolic enzyme targeted by allopurinol. **High-Yield Clinical Pearls for NEET-PG:** * **Drug-Drug Interaction:** Allopurinol inhibits the metabolism of **6-Mercaptopurine (6-MP)** and **Azathioprine**. If co-administered, the dose of these cytotoxic drugs must be reduced by 75% to avoid life-threatening toxicity. * **HLA-B*5801:** Testing for this allele is recommended in certain populations (e.g., Han Chinese, Thai) to prevent **Stevens-Johnson Syndrome (SJS)**, a severe hypersensitivity reaction. * **Acute Gout:** Allopurinol should **never** be started during an acute attack of gout, as a sudden drop in urate levels can mobilize crystals and worsen the inflammation.

Q: Treatment of thyroid storm includes all of the following EXCEPT?

Radioactive iodine. **Explanation:** Thyroid storm is a life-threatening medical emergency characterized by extreme hypermetabolism. The management strategy focuses on inhibiting thyroid hormone synthesis, blocking hormone release, preventing peripheral conversion of T4 to T3, and controlling sympathetic overactivity. **Why Radioactive Iodine (RAI) is the Correct Answer:** Radioactive iodine (I-131) is **contraindicated** in the acute management of thyroid storm. Its mechanism involves the destruction of thyroid parenchyma, which initially causes a massive release of stored thyroid hormones into the circulation. In a patient already in thyroid storm, this "leakage" can worsen the thyrotoxicosis and prove fatal. RAI is reserved for definitive therapy only after the patient has reached a stable, euthyroid state. **Analysis of Other Options:** * **Propranolol (Option A):** Used to control life-threatening sympathetic symptoms (tachycardia, palpitations, tremors). It also uniquely inhibits the peripheral conversion of T4 to T3. * **Hydrocortisone (Option C):** Corticosteroids are used to treat relative adrenal insufficiency associated with severe stress and to inhibit the peripheral conversion of T4 to T3. * **Lugol’s Iodine (Option D):** High doses of stable iodine inhibit the release of thyroid hormones via the **Wolff-Chaikoff effect**. Note: It must be administered *after* starting antithyroid drugs (like PTU) to prevent the iodine from being used as a substrate for new hormone synthesis (Jod-Basedow effect). **NEET-PG High-Yield Pearls:** * **Drug of Choice:** **Propylthiouracil (PTU)** is preferred over Methimazole in thyroid storm because it inhibits both hormone synthesis and the peripheral conversion of T4 to T3. * **Sequence of Treatment:** Always give PTU/Methimazole at least 1 hour **before** giving Lugol’s iodine. * **Supportive Care:** Aggressive cooling (avoiding salicylates as they displace T4 from TBG) and IV fluids are essential.

Q: Dapagliflozin inhibits which glucose transporter?

SGLT-2. **Explanation:**\n\n**Dapagliflozin** belongs to the class of drugs known as **SGLT-2 inhibitors** (Sodium-Glucose Co-transporter 2 inhibitors), often referred to as "Gliflozins" [1].\n\n**Why SGLT-2 is the correct answer:**\nSGLT-2 is a high-capacity, low-affinity transporter located primarily in the **S1 segment of the proximal convoluted tubule (PCT)** of the kidney [3], [4]. It is responsible for approximately 90% of renal glucose reabsorption. By inhibiting SGLT-2, Dapagliflozin promotes **glucosuria** (excretion of glucose in urine), thereby lowering blood glucose levels in patients with Type 2 Diabetes Mellitus [1], [2].\n\n**Why the other options are incorrect:**\n* **GLUT-1:** Found in the blood-brain barrier, RBCs, and fetal tissues; it provides basal glucose uptake.\n* **GLUT-2:** A bidirectional transporter found in the liver, pancreas (beta cells), and the basolateral membrane of the kidney. It has a high Km (low affinity).\n* **GLUT-3:** Primarily found in neurons and the placenta; it has a high affinity for glucose to ensure supply even during hypoglycemia.\n\n**High-Yield Clinical Pearls for NEET-PG:**\n* **Mechanism:** Insulin-independent glucose lowering.\n* **Cardio-Renal Protection:** SGLT-2 inhibitors are now first-line for Heart Failure (HFrEF) and Chronic Kidney Disease (CKD) due to their osmotic diuretic and natriuretic effects.\n* **Side Effects:** Increased risk of **Genital Mycotic Infections** (Candidiasis) and Urinary Tract Infections (UTIs) due to high urinary glucose.\n* **Rare Complication:** Can cause **Euglycemic Diabetic Ketoacidosis (eDKA)**.\n* **Weight Effect:** Leads to modest weight loss (due to calorie loss via glucose) [2].

Q: Conversion of T4 to T3 is inhibited by all except?

Methimazole. ### Explanation The conversion of **Thyroxine (T4)** to the more biologically active **Triiodothyronine (T3)** occurs in peripheral tissues via the enzyme **5'-deiodinase**. Inhibiting this enzyme is a crucial therapeutic strategy in managing severe hyperthyroidism or thyroid storm, as it rapidly lowers the levels of active thyroid hormone. **Why Methimazole is the correct answer:** **Methimazole** (and its prodrug Carbimazole) acts solely by inhibiting the enzyme **thyroid peroxidase**, thereby preventing iodine organification and coupling within the thyroid gland. Unlike Propylthiouracil (PTU), Methimazole has **no effect** on the peripheral conversion of T4 to T3. **Analysis of incorrect options:** * **Propylthiouracil (PTU):** Unlike Methimazole, PTU has a dual mechanism. It inhibits thyroid peroxidase in the gland and **inhibits 5'-deiodinase** in the periphery. This makes it the preferred thioamide in thyroid storm. * **Propranolol:** Beyond its beta-blocking effects (which control tachycardia), high doses of Propranolol inhibit the peripheral conversion of T4 to T3. * **Amiodarone:** This iodine-rich antiarrhythmic inhibits 5'-deiodinase, leading to decreased T3 levels and increased reverse T3 (rT3). * **Glucocorticoids (e.g., Dexamethasone):** Though not listed, these also inhibit peripheral conversion and are used in thyroid storm. ### High-Yield Clinical Pearls for NEET-PG * **Mnemonic for 5'-deiodinase inhibitors:** "**P**eople **P**revent **A**ctive **D**erivative" (**P**TU, **P**ropranolol, **A**miodarone, **D**examethasone). * **Drug of Choice (DOC):** Methimazole is the DOC for hyperthyroidism due to its longer half-life and lower hepatotoxicity, *except* in the first trimester of pregnancy (where PTU is preferred) and thyroid storm (where PTU is preferred). * **Amiodarone** can cause both hypothyroidism (Wolff-Chaikoff effect) and hyperthyroidism (Jod-Basedow phenomenon).

Q: All of the following drugs cause hypertensive crisis in patients with pheochromocytoma, EXCEPT:

Phenoxybenzamine. **Explanation:** In patients with **Pheochromocytoma**, the tumor secretes excessive amounts of catecholamines (epinephrine and norepinephrine). The management of these patients requires careful pharmacological handling to avoid a hypertensive crisis [1], [3]. **Why Phenoxybenzamine is the Correct Answer:** Phenoxybenzamine is a **non-selective, irreversible alpha-blocker**. It is the drug of choice for the preoperative management of pheochromocytoma because it blocks $\alpha_1$ and $\alpha_2$ receptors, preventing catecholamine-induced vasoconstriction [1]. Therefore, it **prevents** rather than causes a hypertensive crisis [2]. **Analysis of Incorrect Options:** * **Propranolol:** If a non-selective beta-blocker is given *before* adequate alpha-blockade, it leads to **unopposed alpha-mediated vasoconstriction** [2]. This causes a paradoxical, life-threatening rise in blood pressure (hypertensive crisis). * **Saralasin:** This is a partial agonist at Angiotensin II receptors. In high-renin states like pheochromocytoma, it can act as an agonist, stimulating the release of catecholamines from the tumor and triggering a crisis. * **Captopril:** While ACE inhibitors are generally safe, they can occasionally cause a profound, unpredictable drop in blood pressure followed by a reactive sympathetic surge, which may trigger a crisis in these patients. **High-Yield Clinical Pearls for NEET-PG:** 1. **Rule of 7-10:** In pheochromocytoma, alpha-blockade (Phenoxybenzamine) must be started **7 to 10 days before** beta-blockade to avoid a hypertensive crisis [1]. 2. **Metyrosine:** An adjuvant drug that inhibits *Tyrosine Hydroxylase* (the rate-limiting enzyme in catecholamine synthesis), used in malignant or inoperable cases [1]. 3. **Glucagon:** Contraindicated in pheochromocytoma as it directly stimulates the tumor to release catecholamines.

Q: Which of the following agents causes relaxation of the uterus to delay labor?

Ritodrine. **Explanation:** The correct answer is **Ritodrine**. **1. Why Ritodrine is correct:** Ritodrine is a **selective $\beta_2$-adrenergic agonist**. In the uterus, $\beta_2$ receptors are responsible for smooth muscle relaxation (tocolysis). When stimulated, these receptors increase intracellular cAMP, which inhibits myosin light chain kinase, leading to decreased uterine contractions. Ritodrine is specifically used as a **tocolytic agent** to delay premature labor [1], allowing time for the administration of corticosteroids (to accelerate fetal lung maturity). **2. Why the other options are incorrect:** * **Tolterodine:** This is a competitive **muscarinic (M3) receptor antagonist** used primarily for overactive bladder and urge incontinence. It acts on the detrusor muscle, not the uterine smooth muscle. * **Oxytocin:** This is a peptide hormone that **stimulates uterine contractions** (oxytocic) [2]. It is used for the induction of labor and the management of postpartum hemorrhage (PPH) [2]. * **Labetalol:** This is a combined **$\alpha_1$ and non-selective $\beta$ blocker**. While it is a first-line agent for pregnancy-induced hypertension (PIH), it does not cause uterine relaxation. **3. NEET-PG High-Yield Pearls:** * **Tocolytic of Choice:** Currently, **Nifedipine** (Calcium Channel Blocker) or **Atosiban** (Oxytocin antagonist) are often preferred over Ritodrine due to a better side-effect profile [1]. * **Side Effects of $\beta_2$ Agonists:** Maternal tachycardia, palpitations, hypokalemia, and pulmonary edema. * **Other Tocolytics:** Magnesium sulfate (neuroprotection) [1], Indomethacin (NSAID - can cause premature closure of ductus arteriosus). * **Mnemonic for Tocolytics:** "**I**t's **N**ot **M**y **T**ime" (**I**ndomethacin, **N**ifedipine, **M**agnesium sulfate, **T**erbutaline/Ritodrine).

Q: Which of the following is a glucocorticoid receptor blocker?

Mifepristone. **Explanation:** **Mifepristone (RU-486)** is the correct answer because it acts as a potent **competitive antagonist at both the progesterone and glucocorticoid (GR) receptors**. While widely known for its use in medical abortions (due to anti-progestational effects), at higher doses, it blocks the glucocorticoid receptor, preventing the binding of endogenous cortisol. This makes it clinically useful for controlling hyperglycemia secondary to hypercortisolism in patients with endogenous **Cushing’s syndrome** who have failed surgery or are not surgical candidates. **Analysis of Incorrect Options:** * **Aminoglutethimide:** This is a steroid synthesis inhibitor. It works by inhibiting the enzyme **cholesterol side-chain cleavage (CYP11A1)**, preventing the conversion of cholesterol to pregnenolone. It does not block the receptor. * **Trilostane:** This is an inhibitor of the **3β-hydroxysteroid dehydrogenase** enzyme. It interferes with the synthesis of cortisol and aldosterone but does not block the glucocorticoid receptor. * **Ketoconazole:** An antifungal that, at high doses, is a non-specific **inhibitor of steroidogenesis** (primarily inhibiting CYP17 and 11β-hydroxylase). It is the most commonly used medical treatment for Cushing’s syndrome but acts via enzyme inhibition, not receptor blockade. **High-Yield NEET-PG Pearls:** * **Mifepristone** is the only drug in this list that is a **receptor blocker**; the others are **synthesis inhibitors**. * **Ketoconazole** is often the first-line medical therapy for Cushing’s disease due to its efficacy and availability. * **Metyrapone** is a selective inhibitor of **11β-hydroxylase**, used primarily for diagnostic testing of the HPA axis and management of hypercortisolism in pregnancy. * **Mitotane** is a "medical adrenalectomy" agent because it is adrenolytic (destroys adrenocortical cells), used in adrenal carcinoma.

Question 1

Which of the following statements is TRUE regarding adrenal suppression caused by steroid therapy?

Accepted Answer

Adrenal suppression is less likely to occur in patients receiving inhaled steroids.

Answer

Adrenal suppression is not associated with atrophy of the adrenal glands.

Answer

Adrenal suppression should be expected in anyone receiving more than 5 mg of prednisolone daily.

Answer

Following cessation of steroid therapy, the stress response normalizes after 8 weeks.

Question 2

Which oral hypoglycemic agent causes the maximum hypoglycemia?

Accepted Answer

Sulfonylureas

Answer

Metformin

Answer

Miglitol

Answer

Pioglitazone

Question 3

Allopurinol specifically inhibits which enzyme?

Accepted Answer

Xanthine oxidase

Answer

Arginase

Answer

Carbamoyl transferase

Answer

Urease

Question 4

Treatment of thyroid storm includes all of the following EXCEPT?

Accepted Answer

Radioactive iodine

Answer

Propranolol

Answer

Hydrocortisone

Answer

Lugol's iodine

Question 5

What is cyproterone acetate used for?

Accepted Answer

Precocious puberty in boys

Answer

Oral contraceptive

Answer

Ovulation inducing agent

Answer

Polycystic ovarian disease (PCOD)

Question 6

Dapagliflozin inhibits which glucose transporter?

Accepted Answer

SGLT-2

Answer

GLUT-3

Answer

GLUT-1

Answer

GLUT-2

Question 7

Conversion of T4 to T3 is inhibited by all except?

Accepted Answer

Methimazole

Answer

Propranolol

Answer

Propylthiouracil

Answer

Amiodarone

Question 8

All of the following drugs cause hypertensive crisis in patients with pheochromocytoma, EXCEPT:

Accepted Answer

Phenoxybenzamine

Answer

Propranolol

Answer

Saralasin

Answer

Captopril

Question 9

Which of the following agents causes relaxation of the uterus to delay labor?

Accepted Answer

Ritodrine

Answer

Tolterodine

Answer

Oxytocin

Answer

Labetalol

Question 10

Which of the following is a glucocorticoid receptor blocker?

Accepted Answer

Mifepristone

Answer

Aminoglutethemide

Answer

Trilostane

Answer

Ketoconazole

Endocrine Pharmacology — MCQs

Endocrine Pharmacology — MCQs

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