A 70-year-old woman with a history of ovarian cancer presents with diarrhea. She completed radiation therapy for her cancer 3 months ago. Physical examination shows cachexia, hyperactive bowel sounds, and generalized pallor. The stools are found to contain blood. A CBC shows decreased hemoglobin (7.8 g/dL) and decreased mean corpuscular volume (70 mm3). Which of the following is the most likely cause of GI bleeding in this patient?