Q. A 54-year-old female is brought to the emergency department with a complaint of severe abdominal pain and distension. She denies any known sick contacts or history of recent travel. The patient has a history of ulcerative colitis and was recently treated with oral corticosteroids and antibiotics for a flare-up of her symptoms. Her past medical history is also significant for hypertension and generalized anxiety disorder. She reports having frequent watery stools for the past week and experiencing pain in the same area for the past three days. On physical examination, her temperature is 38.1 degrees Celsius (100.6 degrees Fahrenheit), blood pressure is 100/70 mm Hg, heart rate is 80/min, respiratory rate is 20/min, and her oxygen saturations are 98 percent on room air. Her cardiopulmonary assessment is within normal limits. The patient is found to have a distended abdomen with a palpable mass in the left lower quadrant. You order a plain abdominal radiograph (see image). You also order a complete blood count and a chemistry panel, as well as stool cultures. Based on this patient’s most likely diagnosis, which of the following is the LEAST likely underlying etiology?