**Core Concept**
The question is testing the side effects of long-term use of diuretic agents, specifically the development of gynaecomastia. Gynaecomastia is a condition characterized by the enlargement of breast tissue in males, often due to an imbalance between estrogen and androgen levels. In the context of diuretics, this is related to their effect on hormone regulation.

**Why the Correct Answer is Right**
Long-term use of spironolactone, a potassium-sparing diuretic, can lead to gynaecomastia. This is because spironolactone acts as an anti-androgen, blocking the effects of testosterone and increasing the levels of estrogen in the body. This hormonal imbalance can cause the growth of breast tissue in males. Spironolactone's mechanism of action involves competitive inhibition of the aldosterone receptor, leading to increased levels of aldosterone in the blood.

**Why Each Wrong Option is Incorrect**
**Option A:** Furosemide is a loop diuretic that can cause electrolyte imbalances and dehydration, but it is not typically associated with gynaecomastia.
**Option B:** Hydrochlorothiazide is a thiazide diuretic that can cause hypokalemia and other electrolyte disturbances, but it does not have a significant anti-androgen effect.
**Option C:** Metolazone is another thiazide-like diuretic that can cause electrolyte imbalances, but it is not known to cause gynaecomastia.

**Clinical Pearl / High-Yield Fact**
It's essential to note that spironolactone is not only used as a diuretic but also as an anti-androgen in the treatment of conditions like hirsutism and acne in females. However, its use requires careful monitoring of hormone levels and potential side effects.

**Correct Answer:** C. Spironolactone.