A patient presents at a physician's office with gait unsteadiness and clumsiness. The patient has a history of type 2 diabetes mellitus, hypertension, and chronic numbness and tingling of the feet and fingertips.
Examination reveals mild distal wasting, glove-and-stocking sensory impairment, absent ankle jerks, brisk knee jerks, and bilateral Babinski reflexes. Gait is slightly ataxic.
CBC reveals a hemoglobin of 10.1 g/dL, WBC of 4,500 cells/mL, MCV 120 fL, and platelets
163,000 cells/ul.
Which of the following is the most important laboratory test for assisting in diagnosis?