A 28-year-old HIV-positive male with a CD4 count of 45/mu L presents with progressive headache, fever, and lethargy over two weeks. Fundoscopic exami…

A 28-year-old HIV-positive male with a CD4 count of $45/\mu L$ presents with progressive headache, fever, and lethargy over two weeks. Fundoscopic examination reveals papilledema. Lumbar puncture is performed, and cerebrospinal fluid (CSF) analysis shows an opening pressure of $280\text{ mm } H_2O$, a white cell count of $20/\mu L$ (predominantly lymphocytes), protein of $85\text{ mg/dL}$, and a low glucose of $35\text{ mg/dL}$. Which of the following is the diagnostic test of choice for confirming the most likely etiology?