A 42-year-old Nigerian man presents with fatigue. He has been exhibiting strange behavior and psychotic symptoms for the past year and is currently being treated with risperidone. He was diagnosed with HIV-1 infection five years ago but has not been following up with treatment.

Upon physical examination, no abnormalities were found. His CD4 count is 20 × 106/l and HIV viral load is > 500,000 copies/ml. Antiretroviral therapy is initiated and the patient is discharged.

One month later, the patient returns with confusion. He is afebrile upon admission and disoriented in time and place but not in person. His CD4 count is now 50 × 106/l and HIV viral load is 503 copies/ml.

Further investigations reveal a hemoglobin level of 95 g/l, WCC of 4.8 × 109/l, neutrophils at 70%, lymphocytes at 20%, and platelets at 400 × 109/l. A CT scan of the brain shows multiple low-density lesions in the right hemisphere.

Cerebrospinal fluid analysis shows a white cell count of 150/ml, glucose level of 3.5 mmol/l, protein level of 0.6 mg/dl, and negative results for cryptococcal antigen, toxoplasmosis PCR, and India ink stain.

Four days later, the patient develops left-sided weakness. A magnetic resonance imaging scan with contrast shows diffuse enhancement of the lesions.

What is the most likely diagnosis?