Clinical Reasoning: A 42-year-old woman with progressive cognitive difficulties and gait imbalance.

A 42-year-old woman presented with 6 months of generalized fatigue, disequilibrium, subjective leg weakness, and difficulty concentrating and completing complex tasks. She had a history of psoriatic arthritis, previously treated with the tumor necrosis factor–α (TNF-α) inhibitor etanercept, which was discontinued about 2 months prior to onset of neurologic symptoms. There was no family history of a similar clinical syndrome. Initial neurologic examination was normal, including normal attention with serial 7s, normal immediate and delayed recall of 3 words, normal muscle tone, full strength with no pronator drift, normal and symmetric reflexes with negative Hoffman signs and flexor plantar responses, intact coordination to finger and foot tapping, unremarkable gait with intact ability to tandem, and negative Romberg sign.