Review Question - QID 210392

A 25-year-old woman presents to her primary care provider for fatigue for the past 6 months. She has tried multiple diets and sleep schedules to improve her condition, but none have succeeded. She has no significant medical history. She is currently taking a multivitamin, folate, B12, iron, fish oil, whey protein, baby aspirin, copper, and krill oil. Her temperature is 98.8°F (37.1°C), blood pressure is 107/58 mmHg, pulse is 90/min, respirations are 13/min, and oxygen saturation is 98% on room air. Laboratory values are seen below.

Hemoglobin: 8 g/dL
Hematocrit: 24%
Leukocyte count: 6,500/mm^3 with normal differential
Platelet count: 147,000/mm^3

Physical exam is notable for decreased proprioception in the lower extremities and 4/5 strength in the patient's upper and lower extremities. Which of the following is the most appropriate next step in management to confirm the diagnosis?