Secondary lactose intolerance — ESEGH MCQ

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EasyLuminal GISecondary lactose intoleranceESEGH

A 23-year-old man develops bloating and diarrhoea after milk following an episode of gastroenteritis. He has no weight loss or rectal bleeding. Coeliac serology is negative and faecal calprotectin is normal. Symptoms improve with lactose avoidance and recur with lactose challenge. What is the most likely diagnosis?

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Correct answer: E — Secondary lactose intolerance

The diagnosis is secondary lactose intolerance. Infectious gastroenteritis can temporarily damage the small-intestinal brush border, reducing lactase activity. Undigested lactose then produces osmotic diarrhoea, bloating and flatulence. The close temporal relationship to milk, improvement with lactose withdrawal and recurrence on rechallenge strongly support this diagnosis. Crohn's disease is less likely given the normal faecal calprotectin, absence of alarm features and lactose-specific pattern. Microscopic colitis causes persistent watery diarrhoea rather than reproducibly milk-triggered symptoms and requires colonic biopsies for diagnosis. Pancreatic exocrine insufficiency more typically causes fat malabsorption, steatorrhoea and weight loss. Negative coeliac serology and the post-infectious, lactose-specific course make coeliac disease unlikely.

Reference: NHS. Lactose intolerance, sections on tests and causes. Updated 8 July 2026. https://www.nhs.uk/conditions/lactose-intolerance/