A 52-year-old man with severe osteoarthritis of the knee is scheduled for total knee replacement under epidural anesthesia. During the procedure, the patient develops sudden onset of severe headache, neck stiffness, and photophobia 45 minutes after epidural catheter placement. His temperature is 38.2°C, blood pressure 145/92 mmHg, and heart rate 110 bpm. CSF analysis shows pleocytosis with predominant lymphocytes, elevated protein (120 mg/dL), and normal glucose. What is the most likely complication?