In HIV-positive patients with AIDS presenting with ring-enhancing brain lesions, which organism is classically implicated and treated with sulfadiazine and pyrimethamine?

• A. Cytomegalovirus
• B. Toxoplasma gondii
• C. Herpes Simplex Virus
• D. Cryptococcus Neoformans

Answer: (B)

A ring-enhancing brain lesion pattern in an HIV/AIDS patient points most strongly to reactivation of Toxoplasma gondii. When the immune system is severely depressed, latent toxoplasma tissue cysts in the brain can reactivate and form focal abscess-like lesions, often in the basal ganglia or at the corticomedullary junction. The favored treatment for toxoplasmic encephalitis is a combination of pyrimethamine and sulfadiazine, with leucovorin (folinic acid) added to protect the bone marrow from pyrimethamine’s folate-blocking effects. This regimen specifically disrupts the parasite’s folate metabolism, which is why it’s effective here. If the patient cannot take sulfa drugs, alternatives such as clindamycin or atovaquone can be used with pyrimethamine, but the classic regimen remains pyrimethamine plus sulfadiazine with leucovorin.

Other pathogens can cause brain imaging abnormalities in AIDS, but their typical presentations differ: cytomegalovirus more often causes ventriculoencephalitis; herpes simplex virus tends to produce a focal temporal lobe encephalitis; cryptococcus usually presents as meningitis or cryptococcomas rather than the classic multiple ring-enhancing lesions seen with toxoplasmosis.