Review Question - QID 215189

A 69-year-old man presents to clinic with 6 months of fatigue, general malaise, and pallor. He has not seen a doctor for the past 3 decades. He does not take any medications and has never had surgery or other medical procedures. He drinks 5 beers per week, does not smoke cigarettes, and denies recreational drug use. Today, his temperature is 97.9°F (36.6°C), blood pressure is 122/68 mmHg, pulse is 92/min, respirations are 14/min, and oxygen saturation is 96% on room air. On physical exam, his abdomen is soft, non-tender, and non-distended. Digital rectal exam demonstrates normal sphincter tone, no palpable masses, and a positive stool guaiac test. A computed tomography scan is shown in Figure A. Which of the following is the most likely cause of these findings?