**Core Concept**
Spironolactone, a potassium-sparing diuretic, is known for its anti-androgenic properties. It acts as an antagonist to the androgen receptor, leading to an imbalance in sex hormones, which can cause gynaecomastia (male breast tissue enlargement).

**Why the Correct Answer is Right**
Spironolactone inhibits the action of dihydrotestosterone (DHT), a potent androgen hormone. This inhibition leads to an increase in the ratio of estrogen to androgen, causing gynaecomastia. The anti-androgenic effects of spironolactone are mediated through its binding to the androgen receptor, preventing the action of DHT on breast tissue.

**Why Each Wrong Option is Incorrect**
* **Option A:** Furosemide is a loop diuretic that does not have significant anti-androgenic effects, making it an unlikely cause of gynaecomastia.
* **Option B:** Hydrochlorothiazide is a thiazide diuretic that can cause electrolyte imbalances but is not associated with gynaecomastia.
* **Option C:** Metolazone is another thiazide-like diuretic that can cause electrolyte imbalances but is not known to cause gynaecomastia.

**Clinical Pearl / High-Yield Fact**
Spironolactone is also used in the treatment of conditions like hirsutism and acne in women due to its anti-androgenic effects, highlighting its unique mechanism of action.

**Correct Answer: C. Spironolactone**