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?