A 28-year-old man presents to the emergency department with a 6-hour history of fever (39.5°C), severe frontal headache, neck stiffness, and photophobia. On examination, he has a positive Kernig sign and Brudzinski sign. His Glasgow Coma Scale is 14/15. Blood cultures have been drawn. CT head (non-contrast) is normal. What is the most appropriate immediate next step in management?