**Core Concept**
The question is testing the association between long-term diuretic use and gynaecomastia, a condition characterized by the enlargement of breast tissue in males. This phenomenon is often related to hormonal imbalances, particularly an excess of estrogen relative to testosterone.

**Why the Correct Answer is Right**
Gynaecomastia can occur due to the anti-androgenic effects of certain diuretics. One such diuretic that can cause this side effect is spironolactone, which is a potassium-sparing diuretic that also has anti-androgenic properties. Spironolactone acts as an antagonist to androgen receptors, leading to an imbalance in sex hormones and resulting in gynaecomastia. This effect is thought to be mediated through the drug's interaction with the mineralocorticoid receptor, which also has some androgen receptor activity.

**Why Each Wrong Option is Incorrect**
**Option A:** Thiazides are a class of diuretics that are not typically associated with gynaecomastia. They work by inhibiting sodium and chloride reabsorption in the kidneys, leading to increased urine production. Thiazides are not known to have significant anti-androgenic effects.

**Option B:** Loop diuretics, such as furosemide, are another class of diuretics that are not commonly associated with gynaecomastia. They work by inhibiting sodium, potassium, and chloride reabsorption in the kidneys, leading to increased urine production. Loop diuretics do not have significant anti-androgenic effects.

**Option C:** Hydrochlorothiazide is a thiazide diuretic that, as mentioned earlier, is not typically associated with gynaecomastia.

**Clinical Pearl / High-Yield Fact**
It's essential to note that spironolactone is not only used for its diuretic effects but also as an anti-androgen in the treatment of conditions such as hirsutism and acne in women. However, its anti-androgenic effects can lead to gynaecomastia in men, highlighting the importance of monitoring for this potential side effect in male patients.

**Correct Answer: C. Spironolactone**