**Core Concept**
Spironolactone is a potassium-sparing diuretic and an antiandrogen that inhibits aldosterone receptors. It is commonly used in hypertension and heart failure but can cause gynecomastia due to its androgenic blockade.

**Why the Correct Answer is Right**
Spironolactone competitively inhibits aldosterone receptors in the kidneys, reducing sodium reabsorption and potassium excretion. Its antiandrogenic properties block androgen receptors, leading to decreased testosterone activity. This results in increased estrogen-like effects, including gynecomastia. The patient's significant breast enlargement over months is a classic sign of this side effect. This mechanism is distinct from other antihypertensives, which do not typically cause gynecomastia.

**Why Each Wrong Option is Incorrect**
Option A: Captopril is an ACE inhibitor that does not affect androgen levels or cause gynecomastia. It may cause hyperkalemia but not breast enlargement.
Option B: Furosemide is a loop diuretic that promotes potassium loss and does not interfere with androgens. It is not associated with gynecomastia.
Option C: Hydrochlorothiazide is a thiazide diuretic that causes mild potassium loss and is not known to cause gynecomastia.

**Clinical Pearl / High-Yield Fact**
Gynecomastia is a hallmark side effect of spironolactone due to its antiandrogenic action. Always consider this in men with breast enlargement, especially on potassium-sparing agents.

✓ Correct Answer: D. Spironolactone