PLE-2420

A 27-year-old woman is brought to the physician because of a 3-day history of visual loss and aching discomfort in her right eye. Examination shows markedly reduced visual acuity in the right eye, with full peripheral visual fields to confrontation. Color vision is decreased in the right eye. The right eye does not react to direct light but has a normal consensual response. Fundoscopic examination shows no abnormalities. She also has an ataxic gait and bilateral Babinski sign. What is the most appropriate next step in diagnosis?