## **Core Concept**
Potassium-sparing diuretics are a class of diuretics that help the body get rid of excess fluid but minimize the loss of potassium. They are often used in the treatment of conditions like hypertension and edema. These drugs act either by antagonizing the aldosterone receptor or by inhibiting epithelial sodium channels.

## **Why the Correct Answer is Right**
The correct answer, **C. Spironolactone**, is a potassium-sparing diuretic because it works by antagonizing the aldosterone receptor. Aldosterone promotes sodium retention and potassium excretion in the collecting ducts of the kidneys. By blocking aldosterone receptors, spironolactone reduces sodium reabsorption and decreases potassium secretion, thus conserving potassium.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Acetazolamide is a carbonic anhydrase inhibitor, which is a different class of diuretics that act by inhibiting the enzyme carbonic anhydrase in the proximal convoluted tubule, leading to increased excretion of sodium, bicarbonate, and water. It does not spare potassium.
- **Option B:** Hydrochlorothiazide (HCTZ) is a thiazide diuretic that acts on the distal convoluted tubule to increase sodium and water excretion but leads to increased potassium excretion, making it a potassium-wasting diuretic.
- **Option D:** Furosemide is a loop diuretic that acts on the Loop of Henle to inhibit sodium, chloride, and water reabsorption. It results in significant potassium loss and is considered a potassium-wasting diuretic.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that potassium-sparing diuretics can lead to hyperkalemia (elevated potassium levels) as a side effect, which can be dangerous. Monitoring potassium levels is crucial when using these medications. Spironolactone, in particular, is also known for its anti-androgenic side effects, such as gynecomastia.

## **Correct Answer:** C. Spironolactone.