A 68-year-old woman is evaluated in the emergency department for sudden-onset, crampy, left-lower-quadrant abdominal pain followed several hours later by passage of bright red blood per rectum. She has hypertension and hyperlipidemia. Current medications are lisinopril and simvastatin.
On physical examination, vital signs are normal. Abdomen is soft and nondistended, with left-lower-quadrant tenderness and no rebound or guarding. Anorectal examination shows scant bright red blood in the rectal vault.
Laboratory testing shows a leukocyte count of 12,000/μL (12 × 109/L) and a blood urea nitrogen level of 24 mg/dL (8.5 mmol/L); other routine laboratory results are normal.
Abdominal and pelvic CT scan shows only segmental thickening of the descending and sigmoid colon.
Which of the following is the most likely diagnosis?
