A 52-year-old man with severe osteoarthritis of the knee is scheduled for total knee replacement under epidural anesthesia. After successful epidural catheter placement at L3–L4 and injection of 15 mL of 0.75% bupivacaine, he develops sudden onset of severe headache, neck stiffness, and confusion within 10 minutes. Blood pressure is 160/95 mmHg, heart rate 110/min, and temperature 38.2°C. What is the most likely diagnosis?