skip to main content

Lactose intolerance — ABIM Board MCQ

Instant feedback + full explanation. One question, done properly.

ModerateGastroenterology/HepatologyLactose intoleranceABIM Board

A 31-year-old develops bloating, gas and watery diarrhea after milk or ice cream. Symptoms resolve with dairy avoidance, there are no alarm features and a hydrogen breath test is positive. What mechanism explains the symptoms?

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

Reveal the answer and explanation

Correct answer: B — Reduced small-intestinal brush-border lactase activity causes osmotic fermentation of lactose

The best answer is “Reduced small-intestinal brush-border lactase activity causes osmotic fermentation of lactose”. Undigested lactose remains osmotically active and is fermented by colonic bacteria, producing hydrogen, gas and diarrhea. Lactase is a brush-border disaccharidase, not a pancreatic enzyme. The reproducible exposure pattern and positive breath test do not fit toxin-mediated, celiac or bile-acid mechanisms.

Reference: NIDDK: Diagnosis of Lactose Intolerance: https://www.niddk.nih.gov/health-information/digestive-diseases/lactose-intolerance/diagnosis