<s>[INST] context: With RBC microcytosis, iron deficiency anemia must be considered. It could be a nutritional deficiency in children and pregnant women, but more likely is due to chronic blood loss in adults. The ferritin concentration is a measure of storage iron because it is derived from the total body storage pool in the liver, spleen, and marrow. About 80% of functional body iron is contained in hemoglobin; the remainder is in muscle myoglobin. Individuals with severe liver disease can have an elevated serum ferritin level because of its release from liver stores. A bone marrow biopsy specimen provides a good indication of iron stores because the iron stain of the marrow shows hemosiderin in macrophages, but such a biopsy is an expensive procedure. Some patients with hemoglobinopathies, such as b-thalassemia, also can have microcytic anemia, but this is far less common than iron deficiency. The serum haptoglobin level is decreased with intravascular hemolysis, but the anemia is normocytic because the iron can be recycled. The serum iron concentration or transferrin level by itself gives no indication of iron stores because, in anemia of chronic disease, the patient's iron level can be normal to low, and the transferrin levels also can be normal to low, but iron stores are increased. Transferrin, a serum transport protein for iron, usually has about 33% iron saturation. [/INST] question: A 54-year-old, previously healthy man has experienced minor fatigue on exertion for the past 9 months. On physical examination, there are no remarkable findings. Laboratory studies show hemoglobin of 11.7 g/dL, hematocrit of 34.8%, MCV of 73 mm3, platelet count of 315,000/ mm3, and WBC count of 8035/ mm3. Which of the following is the most sensitive and cost-effective test that the physician should order to help to determine the cause of these findings? | options: A: Bone marrow biopsy B: Hemoglobin electrophoresis C: Serum ferritin D: Serum haptoglobin | answer: C </s>