A patient with prolonged secretory diarrhoea develops significant hypokalaemia. Which mechanism best explains the potassium loss in chronic diarrhoea?

• A Volume depletion-induced aldosterone secretion enhances colonic K+ secretion in exchange for Na+ reabsorption ✓
• B Passive diffusion of potassium down its concentration gradient across the jejunal mucosa
• C Diarrhoeal fluid contains potassium at a concentration far exceeding plasma, drawn out by solvent drag
• D Reduced colonic transit time prevents passive reabsorption of all filtered potassium

Explanation

The colon responds to volume depletion exactly like the distal nephron: aldosterone upregulates epithelial Na+ channels and the Na+-K+-ATPase, driving vigorous Na+ reabsorption coupled to K+ secretion into the lumen. Chronic diarrhoea therefore produces substantial faecal potassium wasting and hypokalaemia, compounded by metabolic alkalosis from volume contraction. Faecal fluid potassium concentration is actually modest; the loss is driven by enhanced secretion, not by a high concentration gradient.

Reference: Guyton and Hall Textbook of Medical Physiology, 14th ed.

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