Which of the following is considered an osmotic diuretic?

• A. Spironolactone
• B. Triamterene
• C. Mannitol
• D. Furosemide

Answer: (C)

Osmotic diuretics increase urine output by raising the osmolarity of the tubular fluid, so water stays in the filtrate and is excreted. Mannitol fits this mechanism: it is freely filtered but not reabsorbed, so it stays in the nephron and pulls water into the urine, boosting diuresis. The other drugs work by different routes—spironolactone blocks aldosterone in the collecting ducts, causing Na and water loss with potassium retention; triamterene inhibits ENaC channels in the collecting duct; furosemide inhibits the Na-K-2Cl transporter in the thick ascending limb. Thus they are not osmotic diuretics. Mannitol is used clinically for rapid diuresis and to reduce intracranial or intraocular pressure, but requires careful monitoring for fluid and electrolyte shifts.