A 53-year-old man developed patchy numbness in his right arm. The numbness progressed over 10 months to involve his entire right arm, patches of his left arm, and his distal lower extremities. He also developed right-hand incoordination and imbalance. A magnetic resonance image (MRI) of the brain is shown in Figure 1A. A complete blood cell count with a differential; liver function testing; tests for HIV, rapid plasma reagin, serum aquaporin-4 antibody, and antinuclear antibodies; and levels of sodium, calcium, creatinine, glucose, thyroxine, and vitamin B12 were unremarkable. Examinations of the cerebrospinal fluid had normal results twice, including for white blood cell count with a differential, glucose level, total protein level, IgG index, oligoclonal bands, cultures, and cytology testing.