Toxoplasmosis: This is the most common cause of ring-enhancing lesions in HIV patients, particularly those with CD4 counts below 200 cells/μL. Toxoplasmosis is an opportunistic infection caused by the parasite Toxoplasma gondii, which can reactivate in immunocompromised individuals.

Primary Central Nervous System Lymphoma (PCNSL): This is a type of non-Hodgkin lymphoma that arises in the brain and is more common in HIV-infected individuals. PCNSL can present as a ring-enhancing lesion, although it more typically appears as a solid, homogeneously enhancing mass.
Progressive Multifocal Leukoencephalopathy (PML): Caused by the JC virus, PML is an opportunistic infection that affects the white matter of the brain in immunocompromised patients. While it typically does not present as a ring-enhancing lesion, it can have a variety of appearances on imaging, including contrast enhancement in some cases.

Cerebral Abscess: Although less common in HIV patients compared to toxoplasmosis or PCNSL, a cerebral abscess can present as a ring-enhancing lesion and is a medical emergency requiring prompt diagnosis and treatment.
Tuberculous Abscess: In regions where tuberculosis is prevalent, tuberculous abscess of the brain can occur in HIV-infected individuals and may appear as a ring-enhancing lesion on imaging.

Fungal Infections (e.g., Cryptococcosis, Aspergillosis): These can cause ring-enhancing lesions in HIV patients, especially those with very low CD4 counts, but are less common than toxoplasmosis or PCNSL.
Cysticercosis: Caused by the pork tapeworm Taenia solium, neurocysticercosis can present with ring-enhancing lesions, particularly in endemic areas. However, it is less commonly associated with HIV infection compared to other opportunistic infections.