Review Question - QID 102966

A 69-year-old woman presents to the emergency department with severe abdominal pain. She states she was discharged from the hospital 3 days ago after being treated for a kidney infection with intravenous antibiotics. Over the last 5 days, she has developed watery diarrhea and has had constant abdominal pain which has now increased to 10/10 in severity. This has never occurred before. She has a history of hypertension for which she takes amlodipine. She has no surgical history. She lives at home with her husband and dog, plays tennis on the weekends, and does not smoke, drink, or use drugs. The patient's temperature is 103.2°F (39.6°C), blood pressure is 110/60 mmHg, pulse is 112/min, and respirations are 18/min. She has pain with palpation along her lower abdominal quadrants with questionable guarding. Serum laboratory results are as follows:

Hemoglobin: 11.8 g/dL
Hematocrit: 35%
Leukocyte count: 25,000/mm^3
Platelet count: 400,000/mm^3

Na+: 140 mEq/L
Cl-: 90 mEq/L
K+: 3.3 mEq/L
HCO3-: 15 mEq/L
BUN: 16 mg/dL
Glucose: 123 mg/dL
Creatinine: 0.9 mg/dL

Her lactic acid level is 3.6 mmol/L. Figure A illustrates a radiograph of her abdomen. Which of the following is the most likely diagnosis?