<s>[INST] context: This patient has an unexplained pancytopenia. If all three elements (red blood cells, white blood cells, and platelets) are affected, the cause is usually in the bone marrow (although peripheral destruction from hypersplenism can cause pancytopenia as well). In this patient without a history of liver disease or palpable splenomegaly on physical examination, a bone marrow production problem is the most likely culprit. Although B12 deficiency can cause pancytopenia, usually a macrocytic anemia is the most prominent feature; a serum B12 level would be reasonable, but the most productive approach would be to examine the bone marrow. Leukemia can present without leukocytosis (so-called aleukemic leukemia), but the most likely diagnosis would be aplastic anemia. In the elderly patient, myelodysplastic syndrome (MDS) may present with pancytopenia. Decreased levels of erythropoietin can cause decreased RBC production but will not cause pancytopenia. A corticosteroid trial is not warranted until a diagnosis is established. [/INST] question: A 38-year-old woman presents with repeated episodes of sore throat. She is on no medications, does not use ethanol, and has no history of renal disease. Physical examination is normal. Hb is 9.0 g/dL, MCV is 85 fL (normal), white blood cell count is 2000/mL, and platelet count is 30,000/mL. Which of the following is the best approach to diagnosis? | options: A: Erythropoietin level B: Serum B12 C: Bone marrow biopsy D: Liver spleen scan | answer: C </s>