This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Blood, Subject name: Medicine
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? 
Select and explain. 
A. serum B12 level, B. hemoglobin electrophoresis, C. colonoscopy, D. bone marrow aspiration
C: colonoscopy
Answer: C: colonoscopy 
Explanation: 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.