A 69-year-old woman is feeling fatigued. Blood work reveals hemoglobin of 9.0 g/dL (14 g/dL 1 year earlier). She has no other symptoms. Her blood film is shown in Figure given below. Which of the following is the most appropriate next diagnostic test?

The film shows hypochromic, microcytic red cells, suggesting iron deficiency. Although thalassemia can mimic iron deficiency, the normal hemoglobin 1 year earlier makes this unlikely. Anemia of chronic disease is unlikely because there are no signs of such a chronic disease. Thus, blood loss from the GI tract is the most likely cause.