MSFinals-1351

A 68-year-old man presented with fatigue and difficulty breathing. Upon examination, he appeared pale and blood tests showed a hemoglobin level of 62 g/l and a mean corpuscular volume (MCV) of 64 fl. Although he did not exhibit any signs of bleeding, his stool occult blood test (OBT) was positive twice. Despite undergoing upper GI endoscopy, colonoscopy, and small bowel contrast study, all results were reported as normal. What would be the most appropriate next step in investigating this patient?