You are seeing a 27-year-old male in A&E who has presented with rectal bleeding. During the history, the patient informs you that this bleeding has been present for 2 months. The patient also informs you of a painful urge to pass a stool even when the rectum is empty, abdominal pain the lower left quadrant, weight loss and fatigue. On examination you note the presence of finger clubbing, aphthous mouth ulcers and a tenderness in the left lower quadrant. What is the most likely diagnosis?