A 56-year-old man presented to the emergency department with a 3-day history of diarrhea and dull epigastric pain. The diarrhea had responded to antidiarrheal agents. However, the epigastric pain had worsened on the day before presentation and had become sharp, severe, and diffuse.
On physical examination, the patient appeared to be in great distress and was tachycardic, with otherwise normal vital signs. His abdomen was diffusely tender with rebound; bowel sounds were absent.
What is the diagnosis ?