MSFinals-0658

A 70-year-old woman presents to the Emergency Department with severe lower abdominal pain. The pain started yesterday and is increasing in intensity. She has had loose stools for a few days and has been feeling nauseated. She has not vomited. There is no past medical history of note. On examination, there is tenderness and guarding in the left iliac fossa. Bloods: haemoglobin (Hb) 116 g/l; white cell count (WCC) 15 × 109/l.
What is the most likely diagnosis?