This is a primary central nervous system lymphoma. Most commonly associated with HIV/AIDS; pathogenesis involves EBV infection.
Considered an AIDS-defining illness. Variable presentation: confusion, memory loss, seizures. Mass lesion(s) (may be ring-enhancing in immunocompromised patient) on MRI, needs to be distinguished from toxoplasmosis via CSF analysis or other lab tests. Toxo usually has multiple ring enhancing lesions.
Single enhancing lesion points more towards lymphoma than toxo. Toxo is usually multi-ring enhancing lesions.
It cant be Bacterial abscess (Nocardia) because shes taking TMP SMX.
It cant be toxo, because she has one lesion and is also taking TMP SMX which should improve her symptoms.
Glioblastoma is a disease of older individuals
Metastatic disease that mets to the brain is unlikely at this age.
CNS lymphoma is common in HIV AIDS patients, so that is the most likely choice.