**Potassium Sparing Diuretics**

**Core Concept**
Potassium sparing diuretics are a class of medications that inhibit the reabsorption of sodium and water in the collecting duct of the kidneys while sparing potassium. This mechanism is primarily achieved through the blockade of mineralocorticoid receptors or inhibition of the epithelial sodium channel (ENaC).

**Why the Correct Answer is Right**
The correct potassium sparing diuretics act by either blocking the mineralocorticoid receptor (e.g., spironolactone) or inhibiting the epithelial sodium channel (ENaC) (e.g., amiloride and triamterene). This blockade leads to increased sodium excretion, reduced potassium excretion, and a subsequent decrease in blood pressure. Aldosterone antagonists, like spironolactone, also have a direct effect on the mineralocorticoid receptor, which reduces sodium reabsorption and potassium excretion.

**Why Each Wrong Option is Incorrect**
* **Option A:** Furosemide is a loop diuretic that acts on the thick ascending limb of the loop of Henle, increasing sodium and water excretion while also promoting potassium excretion. It is not a potassium sparing diuretic.
* **Option B:** Hydrochlorothiazide is a thiazide diuretic that acts on the distal convoluted tubule, increasing sodium and water excretion while also promoting potassium excretion. It is not a potassium sparing diuretic.
* **Option D:** Metolazone is a thiazide-like diuretic that acts on the distal convoluted tubule, increasing sodium and water excretion while also promoting potassium excretion. It is not a potassium sparing diuretic.

**Clinical Pearl / High-Yield Fact**
When using potassium sparing diuretics, monitor patients for hyperkalemia, especially in those with renal impairment or those taking other medications that increase potassium levels. Regular serum potassium checks are essential to prevent hyperkalemia.

**Correct Answer:** C. Triamterene is a potassium sparing diuretic that inhibits the epithelial sodium channel (ENaC) in the collecting duct of the kidneys.