A 26-year-old man presented to the outpatient clinic of internal medicine with a three-month history of diarrhoea. He had loose, watery stools without mucous or blood up to five times a day. He had no fever or weight loss. His medical history was unremarkable and at presentation he was not on any medication. Social anamnesis revealed that he had homosexual anal receptive contacts. At physical examination, the abdomen had positive bowel sounds and was soft and non-tender without palpable masses or organomegaly. Additional general physical examination also reveiled no abnormalities. Renal, liver and electrolyte profiles, blood count and white blood cell differential were within normal ranges. Diagnostic tests revealed no evidence for hyperthyroidism or celiac disease. Faecal cultures were negative for bacteria and parasites. Sexually transmitted disease screening, including a rectal swab for C. trachomatis, and HIV screening were negative. Finally, the patient underwent colonoscopy. No abnormalities were seen macroscopically, but histological examination of random colon biopsies showed colonic crypts with a thick ‘fuzzy’ brush border (figures 1 and 2).