A 35-year-old woman with established SLE (ANA+, anti-dsDNA+, low C3/C4) on hydroxychloroquine and prednisolone 10 mg/day presents with acute-onset severe headache, photophobia, and neck stiffness. CSF analysis shows lymphocytic pleocytosis (120 cells/µL, 90% lymphocytes), elevated protein (85 mg/dL), normal glucose, and negative bacterial/viral cultures. MRI brain is normal. What is the most appropriate next step?