Which organism is a common cause of ring-enhancing brain lesions most commonly in AIDS patients?

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

Answer: (A)

Ring-enhancing brain lesions in an AIDS patient are most classically due to reactivation of latent Toxoplasma gondii infection. When CD4 counts are very low, toxoplasma can reactivate in the brain and produce multiple focal areas of inflammation and necrosis. The inflammatory response around these foci creates a ring of enhancement on imaging, which is why these lesions appear with a bright rim and a relatively dark center on CT or MRI.

Toxoplasma gondii is a protozoan parasite commonly acquired from undercooked meat or cat feces; many people carry latent infection, and immune suppression allows it to reactivate in the CNS, presenting with headaches, fever, confusion, seizures, or focal neurologic deficits. In this context, toxoplasmosis is the most frequent cause of ring-enhancing lesions.

Other organisms can cause brain lesions but fit different patterns. Cryptococcus neoformans often presents as meningitis with signs like meningitis symptoms and elevated opening pressure, though cryptococcomas can occur. Mycobacterium tuberculosis can form tuberculomas that may ring-enhance, but this is less typical as the most common cause in AIDS for ring-enhancing lesions. JC virus causes progressive multifocal leukoencephalopathy, which generally shows non-enhancing demyelinating lesions rather than ring-enhancing masses.

So the best-fitting explanation for ring-enhancing brain lesions in this setting is reactivated Toxoplasma gondii encephalitis.