Which of the following diuretics is a loop diuretic?

• A. Furosemide
• B. Hydrochlorothiazide
• C. Spironolactone
• D. Acetazolamide

Answer: (A)

Loop diuretics block the Na-K-2Cl cotransporter in the thick ascending limb of the loop of Henle, leading to substantial excretion of sodium, chloride, and water. Furosemide is the classic loop diuretic that carries out this action, which is why it’s used to mobilize edema in heart failure, liver disease, and kidney disease, and in certain urgent settings. The other drugs act at different sites: hydrochlorothiazide inhibits the Na-Cl transporter in the distal tubule (thiazide diuretic), spironolactone blocks aldosterone in the collecting duct (potassium-sparing diuretic), and acetazolamide inhibits carbonic anhydrase in the proximal tubule (carbonic anhydrase inhibitor). Thus, furosemide is the loop diuretic.