**Question:** Regarding furosemide, which of the following statements is true?

A. Furosemide is a potassium-sparing diuretic.
B. Furosemide primarily targets the proximal tubule of the nephron.
C. Furosemide is contraindicated in patients with severe renal impairment.
D. Furosemide does not affect calcium homeostasis.

**Core Concept:**
Furosemide is a loop diuretic, a type of synthetic antibiotic diuretic used to increase urine output and reduce fluid retention. Loop diuretics, including furosemide, work in the distal convoluted tubule and collecting duct of the nephron, inhibiting the Na+/K+ ATPase pump and promoting the secretion of chloride ions, sodium ions, and water, leading to diuresis (increased urine output).

**Why the Correct Answer is Right:**

B. Furosemide primarily targets the distal convoluted tubule and the collecting duct of the nephron, not the proximal tubule. It inhibits the Na+/K+ ATPase pump, leading to diuresis (increased urine output).

**Why Each Wrong Option is Incorrect:**

A. Furosemide is a potassium-losing agent, meaning it increases potassium excretion in the urine. This contradicts the statement that furosemide is a potassium-sparing diuretic.

C. Although furosemide may cause transient hypokalemia, it does not mean it is contraindicated in severe renal impairment. It is usually administered with potassium supplementation to counteract the hypokalemia.

D. Furosemide does affect calcium homeostasis by increasing urinary calcium excretion due to the diuretic action on the distal nephron (which influences calcium reabsorption).

**Clinical Pearl:**
Furosemide is a valuable tool in the management of fluid overload, hypertension, and edema, particularly in patients with congestive heart failure (CHF), chronic kidney disease (CKD), or cirrhosis. However, its use should be judicious in patients with severe renal impairment, as it may exacerbate hypokalemia and calcium abnormalities. In such cases, potassium supplementation and close monitoring are essential to ensure safety and efficacy.