A 72-year-old woman is re-evaluated for iron deficiency anemia and intermittent dark-colored stools. Since the incidental discovery of iron deficiency anemia 3 months ago, she has been evaluated with upper endoscopy that showed a large hiatal hernia and specks of old blood in the stomach but no clear bleeding source. Colonoscopy was normal. Her only medication is iron sulfate.

On physical examination, temperature is 37.0 °C (98.6 °F), blood pressure is 122/78 mm Hg, pulse rate is 72/min, and respiration rate is 18/min. Abdominal examination findings are normal.

Laboratory studies show a hemoglobin level of 11.5 g/dL (115 g/L) and a mean corpuscular volume of 77 fL. A guaiac fecal occult blood test is positive.

Which of the following is the most appropriate diagnostic test to perform next?