A 38-year-old HIV-positive man with CD4 count 80 cells/uL presents with fever, headache, and left hemiparesis. MRI shows multiple ring-enhancing lesions in the basal ganglia with marked edema. Empirical therapy is started. Which organism is the most likely cause?

• A Toxoplasma gondii ✓
• B Cryptococcus neoformans
• C Mycobacterium tuberculosis
• D JC virus

Explanation

Multiple ring-enhancing basal ganglia lesions in an AIDS patient with CD4 below 100 cells/uL is the classic presentation of cerebral toxoplasmosis, caused by reactivation of Toxoplasma gondii. Lesions show central necrosis with free tachyzoites and encysted bradyzoites at the periphery. Cryptococcus causes meningitis rather than focal ring lesions, JC virus causes non-enhancing demyelination in PML, and tuberculosis typically gives a single ring lesion with meningeal involvement.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

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