A 40-year-old man has 2-month history of unsteadiness while walking that is associated with lower extremity numbness. He underwent bariatric surgery 2 years ago. His laboratory work up Platelets = 180,000/ uL (N: 150,000-400,000) WBC = 4000/ uL (N: 4000-10.000) Hemoglobin = 9.5 g/dl. (N: 13-17) MCV = 110 f (N: 80-. What is the single test to work up his presentation?