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Mannitol Osmotic Diuresis — FRCA Primary MCQ

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HardPhysiology - RenalMannitol Osmotic DiuresisFRCA Primary

Mannitol is administered for acute intracranial hypertension. Which renal mechanism contributes directly to its diuretic effect?

Educational content. Not a substitute for clinical judgement or local policy.

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Correct answer: B — Filtered mannitol raises tubular-fluid osmolality and reduces passive water reabsorption

The best answer is “Filtered mannitol raises tubular-fluid osmolality and reduces passive water reabsorption”. Mannitol is filtered and poorly reabsorbed, increasing tubular-fluid osmolality and opposing water reabsorption. It does not inhibit NKCC2, carbonic anhydrase, mineralocorticoid receptors or V2 receptors; those mechanisms belong to loop diuretics, acetazolamide, spironolactone and vaptans respectively. Its initial extracellular volume expansion also matters clinically.

Reference: RCoA, Primary FRCA syllabus: https://www.rcoa.ac.uk/sites/default/files/documents/2026-01/Primary-FRCA-Syllabus.pdf; Diuretics and the kidney: https://pmc.ncbi.nlm.nih.gov/articles/PMC9125415/