Final answer:
The likely diagnosis is cryptococcal meningitis, a fungal infection caused by Cryptococcus neoformans or Cryptococcus gattii. It is a common opportunistic infection in HIV/AIDS patients.
Step-by-step explanation:
b) Cryptococcal meningitis
In an HIV patient with altered mental status, EBV DNA in the CSF, and weakly ring-enhancing periventricular mass, the likely diagnosis is cryptococcal meningitis. Cryptococcal meningitis is a fungal infection caused by Cryptococcus neoformans or Cryptococcus gattii. It is a common opportunistic infection in patients with HIV/AIDS, particularly those with low CD4 cell counts.
EBV DNA in the CSF and weakly ring-enhancing periventricular mass are not typical findings in toxoplasmosis or cytomegalovirus (CMV) encephalitis. Progressive multifocal leukoencephalopathy (PML) is caused by reactivation of John Cunningham virus (JCV) in patients with impaired immune function, and is usually associated with JC virus DNA in the CSF and characteristic white matter lesions on neuroimaging.