A 35-year-old woman from Kolkata presents on day 8 of illness with persistent high fever (39–40°C), severe headache, and altered mental status. She is confused and disoriented. On examination, she has neck stiffness, positive Kernig sign, and hepatosplenomegaly. Blood and CSF cultures are pending. CSF analysis shows: protein 120 mg/dL, glucose 35 mg/dL (serum glucose 95 mg/dL), WBC 250/μL (80% lymphocytes, 20% neutrophils). She was started on ceftriaxone 2 g IV 12-hourly 2 days ago for presumed enteric fever. What is the most likely diagnosis and the critical additional intervention required?