Review Question - QID 214753

A 50-year-old man presents to the emergency department because of worsening fatigue, malaise, and headache for the past 3 weeks. He was not concerned about it until he developed blurry vision 1 week ago. He has fevers and night sweats throughout this time but no cough or hemoptysis. He was diagnosed with human immunodeficiency virus 3 years ago and is not adherent to treatment as he cannot afford his medications. He denies any recent travel or sick contacts. His temperature is 101.3°F (38.5°C), blood pressure is 125/85 mmHg, pulse is 92/min, and respirations are 16/min. On physical exam, he endorses pain on passive neck flexion and decreased range of motion of the cervical spine. Funduscopic exam reveals papilledema. Cardiopulmonary and neurologic exams reveal no further abnormalities. A lumbar puncture with cerebrospinal fluid (CSF) analysis and blood tests are obtained which demonstrate:

Hemoglobin: 13 g/dL
Leukocyte count: 4,000/mm^3
CD4+ T-cells: 70/mm^3

Lumbar Puncture:
Opening pressure: 320 mm H2O
Leukocyte count: 25/mm^3 (64% lymphocytes)
Protein concentration: 160 mg/dL
Glucose concentration: 30 mg/dL

A CT scan of the head is unremarkable. Which of the following diagnostic tests of the cerebrospinal fluid (CSF) is the most appropriate next step in management?