You are answering a USMLE-style medical question. Question: A 56-year-old woman comes to the physician because of a 2-week history of fatigue and painless bruising over her arms and trunk. She has also had several episodes of nosebleeds that resolved with compression after a few minutes. She recently completed treatment for a urinary tract infection. She has had no changes in her weight. She has type 2 diabetes mellitus and hypertension. Her last menstrual cycle was 5 years ago. She does not smoke or drink alcohol. Home medications include metformin, amlodipine, and enalapril. Her vital signs are within normal limits. Physical examination shows pale conjunctivae. There are ecchymoses and petechiae over the upper extremities, chest, and back. There is no lymphadenopathy. The remainder of the physical examination is unremarkable. Laboratory studies show: Hemoglobin 8.7 mg/dL Leukocyte count 1100/mm3 Platelet count 54,000/mm3 Reticulocyte count 0.1% Mean corpuscular volume 93 μm3 Serum Total bilirubin 1.1 mg/dL LDH 80 U/L Which of the following is most likely to confirm the diagnosis?" Options: A) Serum transferrin level B) Bone marrow biopsy C) Peripheral blood smear D) Serum porphobilinogen level Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.