NBME Microbiology Practice Test

The key concept is how toxoplasma encephalitis presents in severely immune-compromised patients. In AIDS, when CD4 counts fall well below 100, latent Toxoplasma gondii infection can reactivate in the brain. The parasite forms focal areas of necrosis surrounded by inflammatory tissue, which on contrast-enhanced imaging show up as multiple ring-enhancing lesions. These lesions are classically located at the gray–white matter junction and in the basal ganglia, and patients often present with headaches, confusion, focal neurologic deficits, or seizures due to the localized inflammation.

This pattern is most characteristic of toxoplasma in this setting, whereas other organisms tend to cause different patterns. Cryptococcus usually causes meningitis with meningismus and hydrocephalus rather than multiple ring-enhancing parenchymal lesions. Cytomegalovirus more often produces ventriculoencephalitis or periventricular enhancement, not the typical multiple ring-enhancing masses. Candida is less commonly a cause of focal brain ring-enhancing lesions in AIDS. Thus, reactivation of Toxoplasma gondii best explains the imaging and clinical picture in an AIDS patient with these brain lesions.