Use the fоllоwing vignette tо аnswer questions 20 - 22. A 65-yeаr-old mаn is evaluated in the emergency department for painless bright red blood per rectum that began 6 hours ago. The patient reports a vague history of constipation and abdominal pain. He takes no medications. On physical examination, temperature is 38.6°C (101.5°F), blood pressure is 130/78 mm Hg, pulse rate is 96/min, and respiration rate is 18/min. Abdominal palpation reveals lower left quadrant pain with guarding. Rectal examination discloses no external hemorrhoids; bright red blood is noted in the rectal vault. There is no evidence of abscess or colovesical fistula. Laboratory studies show a hemoglobin level of 10.4 g/dL (normal, 14-17 g/dL), a leukocyte count of 14,000/µL (normal, 4000-10,000/μL). Which of the following is the most likely cause of this patient’s GI bleeding?
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