Anti-NMDA Receptor Encephalitis — Extreme Delta Brush

A 22-year-old woman presents with a 3-week history of fever, headache, and progressive behavioral changes including paranoid delusions and agitation. Over the past week, she has developed involuntary orofacial movements and seizures. EEG shows the pattern marked **A** in the diagram — high-amplitude rhythmic delta waves with superimposed beta activity. CSF analysis reveals lymphocytic pleocytosis with oligoclonal bands. Serum and CSF anti-NMDA receptor IgG antibodies are positive. Which of the following is the MOST IMPORTANT next step in management?