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Loop Diuretics Site of Action — FRCA Primary MCQ

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HardPhysiology - RenalLoop Diuretics Site of ActionFRCA Primary

Furosemide is administered to a patient with pulmonary oedema. Which electrolyte effect follows most directly from NKCC2 inhibition in the thick ascending limb?

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Correct answer: E — Lower lumen-positive voltage with increased calcium and magnesium excretion

The best answer is “Lower lumen-positive voltage with increased calcium and magnesium excretion”. NKCC2 activity and potassium recycling normally create a lumen-positive potential in the thick ascending limb that drives paracellular calcium and magnesium reabsorption. Furosemide reduces this voltage, increasing urinary calcium and magnesium losses. ENaC blockade is the mechanism of amiloride, while AQP2 trafficking is regulated by vasopressin.

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/