A 58-year-old woman presented to the emergency room with a history of 4 days of dark loose stool and a 24-hr history of worsening abdominal pain. There was no history of prior abdominal surgery or ulcer disease. On abdominal exam there was diffuse 3+ tenderness, guarding and 1+ rebound. Bowel sounds were hypoactive. She was afebrile, WBC was 13K without a shift. Hemoglobin and hematocrit were low normal. Her normal abdominal upright film is shown.