You are seeing a 40-year-old male in A&E who has presented with abdominal pain. This patient informs you that the pain came on suddenly several hours ago and is getting worse, which he states started in the epigastric region but has now spread to the whole abdomen. On examination you note that this patient has a temperature of 38.6 degrees, a heart rate of 123, rebound tenderness and tenderness on percussion. Bowel sounds are absent. What is the most likely diagnosis?