A 28-year-old woman with HIV (CD4 count 85 cells/μL, on ART for 3 months) presents with headache, fever, and neck stiffness for 5 days. CSF analysis shows: protein 120 mg/dL, glucose 35 mg/dL (serum glucose 95 mg/dL), WBC 15 cells/μL (85% lymphocytes), negative Gram stain and bacterial culture. India ink stain is pending. What is the most appropriate immediate next step?