A 52-year-old female is admitted to the hospital with a three day history of nausea and vomiting. She has a past medical history of hypertension, hyperlipidemia, and obesity currently three months status post Roux-en-Y bypass surgery. Upon initial evaluation of the patient the nurse practitioner learns that the patient is experiencing numbness of her hands and feet, difficulty walking and maintaining balance and weakness. On physical exam the patient is found to have positive Rhomberg and Babinski signs. Initial laboratory studies included a CBC which revealed WBCs of 6.7, hemoglobin 10.5, MCV 115, and platelets 250,000. Given these results in the findings and physical exam the next appropriate laboratory study would be: A. Vitamin B12 level B. Bone marrow biopsy C. Peripheral smear D. Electrolyte panel