An HIV-positive patient with six ring-enhancing lesions in the brain and surrounding edema receives treatment of choice for the suspected infection. Which regimen is preferred?

• A. Radiation therapy
• B. Ventricular shunt placement
• C. Sulfadiazine and pyrimethamine
• D. Trimethoprim/sulfamethoxazole

Answer: (C)

In an HIV-positive patient with multiple ring-enhancing brain lesions and surrounding edema, toxoplasma encephalitis is the most likely infectious cause. The preferred treatment is pyrimethamine plus sulfadiazine, with folinic acid (leucovorin) added to protect the host’s bone marrow from pyrimethamine’s toxicity. Pyrimethamine inhibits the parasite’s dihydrofolate reductase, blocking folate synthesis necessary for parasite replication, while sulfadiazine provides an additional antifolate mechanism. The combination is then supplemented with leucovorin to prevent hematologic toxicity in the patient. Radiation therapy or ventricular shunt placement would not address this infectious process. TMP-SMX is a valid alternative if the first-line regimen is unavailable or not tolerated, but pyrimethamine-sulfadiazine with leucovorin remains the standard initial therapy.