What is the most likely diagnosis for an HIV-positive patient with multiple ring-enhancing lesions on a CT scan?

In an HIV-positive patient presenting with multiple ring-enhancing lesions on a CT scan, cerebral toxoplasmosis is the most likely diagnosis. This is primarily due to the common nature of this condition in immunocompromised individuals, particularly those with CD4 counts below 200 cells/mm³. Toxoplasmosis is caused by the parasite Toxoplasma gondii, which is often reactivated in patients with advanced HIV/AIDS and can lead to significant neurological symptoms.

The characteristic appearance of multiple ring-enhancing lesions, especially when they are located in the basal ganglia or corticomedullary junction, aligns well with cerebral toxoplasmosis. These lesions typically appear as enhancing areas with surrounding edema on imaging, and this pattern is what makes toxoplasmosis the leading diagnosis in such scenarios.

In contrast, while primary CNS lymphoma can also present as a similar imaging finding, it usually appears as a solitary lesion rather than multiple lesions, and it has a different clinical progression and associated symptoms. Cryptococcal infection often presents with meningoencephalitis rather than focal lesions, and bacterial abscesses are typically few in number and often associated with a specific infectious source. Thus, the details provided strongly favor cerebral toxoplasmosis as the most probable diagnosis in