Review Question - QID 210498

A 72-year-old woman presents to the emergency room with dark stools. She first noticed that her stool appeared dark and tarry the day prior to presentation. She had 3 similar bowel movements over the past 24 hours. Her bowels movements are normally brown, solid, and occur once per day. Her past medical history is notable for gastroesophageal reflux disease, hypertension, hyperlipidemia, diabetes, and stage 3 chronic kidney disease. She takes omeprazole, enalapril, atorvastatin, and insulin. She does not smoke, and she drinks 6-8 glasses of wine per week. Her family history is notable for Bernard-Soulier disease in her paternal uncle and rectal cancer in her father. Her temperature is 98.6°F (37°C), blood pressure is 108/61 mmHg, pulse is 105/min, and respirations are 21/min. A fecal occult blood test is positive. Chest auscultation reveals normal breath sounds bilaterally and a harsh 3/6 crescendo-decrescendo murmur best heard at the upper right sternal border. Laboratory analysis is shown below:

Hemoglobin: 10.4 g/dL
Hematocrit: 30%
Leukocyte count: 7,000/mm^3 with normal differential
Platelet count: 240,000/mm^3

A notable finding on colonoscopy is shown in Figure A. Which of the following is the underlying cause of this patient's symptoms?