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ADH response to hypovolaemia — FRCA Primary MCQ

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ModeratePhysiologyADH response to hypovolaemiaFRCA Primary

After bowel preparation and fasting, a patient is tachycardic and has concentrated urine before surgery. Which hormonal response most directly increases free water reabsorption?

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

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Correct answer: E — Increased vasopressin acting on V2 receptors

Vasopressin acts on V2 receptors in the collecting duct to insert aquaporin-2 channels and increase free water reabsorption. Aldosterone increases sodium reabsorption and potassium secretion, but is not the principal free-water hormone. Atrial natriuretic peptide and reduced renin would oppose volume retention. Hypovolaemia can stimulate vasopressin even when osmolality is not markedly raised.

Reference: Ganong Review of Medical Physiology; Primary FRCA physiology texts