An eight-year-old boy had abdominal pain, fever with bloody diarrhoea for 18 months. His height is 100 ems and weight is 14.5kg. Stool culture was negative for known enteropathogens. The sigmoidoscopy was normal. During the same period, the child had an episode of renal colic and passed urinary gravel. The Mantoux test was 5 x 5 mm. The most probable diagnosis is –