question: Q:A 39-year-old woman presents with progressive weakness, exercise intolerance, and occasional dizziness for the past 3 months. Past medical history is unremarkable. She reports an 18-pack-year smoking history and drinks alcohol rarely. Her vital signs include: temperature 36.6°C (97.8°F), blood pressure 139/82 mm Hg, pulse 98/min. Physical examination is unremarkable. Her laboratory results are significant for the following:
  Hemoglobin 9.2 g/dL
  Erythrocyte count 2.1 million/mm3
  Mean corpuscular volume (MCV) 88 μm3
  Mean corpuscular hemoglobin (MCH) 32 pg/cell
  Leukocyte count 7,500/mm3
  Which of the following is the best next step in the management of this patient’s condition?? 
  {'A': 'Serum ferritin level', 'B': 'C-reactive protein (CRP)', 'C': 'Reticulocyte count', 'D': 'Direct antiglobulin test', 'E': 'Bone marrow biopsy'},