Progressive multifocal leukoencephalopathy
(Choice C) Fluconazole is occasionally used for primary prophylaxis against coccidioidomycosis (for those in endemic areas with newly converted serology). Coccidioidomycosis primarily manifests with pulmonary disease but occasionally causes meningitis. However, multiple ring-enhancing lesions would be atypical. Fluconazole is also used as secondary prophylaxis (suppressive therapy) for those with a history of cryptococcal meningitis (when CD4 count is still low); fluconazole is not used as primary prophylaxis for Cryptococcus, and Cryptococcus does not typically cause ring-enhancing brain lesions.