You are seeing a 64-year-old male in A&E who has presented with abdominal pain and rectal bleeding. The patient informs you that this pain came on yesterday that started in the centre of his abdomen and is now radiating into the left lower quadrant. On examination you note a tenderness in the left lower quadrant as well as a palpable abdominal mass. During observations you note the patient has a temperature of 38.3 degrees. What is the most likely diagnosis?