**Core Concept**
Aldosterone antagonists are a class of medications that inhibit the action of aldosterone, a mineralocorticoid hormone that plays a crucial role in regulating electrolyte balance and blood pressure. Aldosterone antagonists work by blocking the mineralocorticoid receptor, thereby reducing sodium reabsorption and potassium excretion in the kidneys.

**Why the Correct Answer is Right**
Spironolactone is a potassium-sparing diuretic that acts as an aldosterone antagonist by binding to the mineralocorticoid receptor (MR) with high affinity. This binding prevents aldosterone from interacting with the MR, resulting in decreased sodium reabsorption and increased potassium excretion in the collecting ducts of the kidneys. This mechanism helps to reduce blood pressure and alleviate symptoms of heart failure.

**Why Each Wrong Option is Incorrect**
**Option A:** None of the other options are aldosterone antagonists.
**Option B:** Hydrochlorothiazide is a thiazide diuretic that acts by inhibiting sodium reabsorption in the distal convoluted tubule, but it is not an aldosterone antagonist.
**Option C:** Furosemide is a loop diuretic that acts by inhibiting sodium, potassium, and chloride reabsorption in the thick ascending limb of the loop of Henle, but it is not an aldosterone antagonist.

**Clinical Pearl / High-Yield Fact**
Aldosterone antagonists like spironolactone are often used in the treatment of resistant hypertension and heart failure, particularly in patients with renal impairment.

**Correct Answer:** C. Spironolactone.