An elderly gentleman, with background of stroke, hypertension, ischaemic heart disease, peripheral vascular disease, mild cognitive impairment and chronic obstructive lung disease, was admitted to hospital due to 3 days history of bloody diarrhoea. He was recently discharged from hospital around 5 days ago, after being treated with a course of antibiotic for his infective exacerbation of chronic obstructive lung disease, with 1 week of hospital stay. He was well after discharged, but started to develop bloody diarrhoea up to 5-6 times a day since 3 days ago, accompany with slight minimal general abdominal discomfort. On examination, there was minimal tenderness around abdomen. Blood tests showed normal full blood count, with slight elevated inflammatory marker (CRP). X-ray of abdomen and chest were normal. What is your working diagnosis ? What are the investigation and treatment for this patient ?