An 18 year old male presented to the outpatient medical clinic for evaluation of diarrhoea and abdominal discomfort. The patient first noted mild abdominal discomfort and three loose bowel movements per day 1 week prior to evaluation. Two days prior to evaluation he noted intermittent, crampy periumbilical abdominal pain. He denied drinking well water, fever, blood in the stool, relation of the pain to meals, dysuria, or hematuria. On examination the patient was afebrile and had normal vital signs. The abdominal examination was notable for mild lower abdominal tenderness. The faecal examination demonstrated a greenish watery stool that was negative for occult blood. Laboratory evaluation included a normal white blood cell count, hematocrit, and platelet count. Examination of the faeces microscopically was remarkable for the presence of white blood cells. please tell me i am a trainee biomedical scientist .i have to submit this case study . what is diagnosis?