A 64-year-old man presents with fatigue and dyspnea on exertion. CBC: hemoglobin 9.2 g/dL, MCV 72 fL. Iron studies: serum iron 32 mcg/dL (low), TIBC 480 mcg/dL (high), ferritin 9 ng/mL (low). He has no known menstrual loss (he is male) and no obvious bleeding. According to current AGA and ACG guidance on iron-deficiency anemia, which is the MOST appropriate next diagnostic step?
