A 13-year-old boy has 6 months of abdominal pain and diarrhoea. On examination he has aphthous ulcers on the palate and tender erythematous nodules over the shins. Other systems are normal. What is the most probable diagnosis?

Answer: A. Crohn's disease

Crohn's disease

Chronic (6-month) diarrhoea and abdominal pain in a teenager, with aphthous (oral) ulcers and erythema nodosum (tender erythematous shin nodules), points to Crohn's disease - a chronic inflammatory bowel disease that can affect any part of the gut and frequently has extraintestinal manifestations.

Why Crohn's fits

• Aphthous ulcers reflect mouth-to-anus involvement characteristic of Crohn's.
• Erythema nodosum is a recognised skin manifestation of IBD.
• The chronic time course argues against an acute infective cause.

Chronic diarrhoea by mechanism

• This is an inflammatory chronic diarrhoea (mucosal inflammation/ulceration), the category that also includes ulcerative colitis and radiation/ischaemic colitis.

Management principle

Inflammatory bowel disease is treated by addressing the inflammation (aminosalicylates, corticosteroids for flares, immunomodulators/biologics) under specialist care - not merely controlling symptoms.