Countercurrent Multiplier Location — FRCA Primary MCQ
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Correct answer: D — The loop of Henle, through differential water and solute permeability
The best answer is “The loop of Henle, through differential water and solute permeability”. Countercurrent multiplication occurs in the loop of Henle: the water-permeable descending limb and solute-reabsorbing, relatively water-impermeable ascending limb generate the medullary gradient. The vasa recta acts mainly as a countercurrent exchanger that limits washout. Collecting-duct water reabsorption uses, rather than creates, the established gradient.
Reference: Royal College of Anaesthetists, Primary FRCA Syllabus 2021: https://www.rcoa.ac.uk/sites/default/files/documents/2026-01/Primary-FRCA-Syllabus.pdf