Okay, let's tackle this question. The patient is an untreated AIDS patient with a low CD4+ count (180 cells/mm³), which is a key point. He has symptoms like severe headache, mental confusion, ataxia, and retinochoroiditis. The CT scan shows focal brain lesions, and he's been homeless living under a bridge for two years. High IgG levels suggest a chronic infection.

First, I need to think about opportunistic infections in AIDS patients with low CD4 counts. Common ones include Toxoplasma gondii, Cryptococcus neoformans, JC virus (PML), and CMV. But the presence of retinochoroiditis and focal brain lesions makes me think of Toxoplasma. Toxoplasmosis often presents with ring-enhancing lesions in the brain and can cause ocular issues. The high IgG indicates prior infection, which is reactivated due to immunosuppression.

The patient's living conditions under a bridge might mean exposure to cats, which are the primary hosts for Toxoplasma. If he had a previous infection (as IgG shows), his weakened immune system now can't control it, leading to reactivation.

Other options like Cryptococcus usually present with a chronic meningitis picture and might have a fungal meningitis with headache, but retinochoroiditis isn't typical. PML is caused by JC virus and leads to demyelination, but it doesn't have retinochoroiditis. CMV can cause retinitis, but the brain lesions in CMV are different.

So the most likely answer is Toxoplasma gondii reactivation from prior infection. The high IgG supports this as a reactivation rather than a new infection. The living conditions (homeless, possibly poor hygiene) would increase the risk of Toxoplasma exposure through cat feces or undercooked meat. The key here is the combination of brain lesions, retinochoroiditis, and high IgG in an AIDS patient.

**Core Concept**
The question evaluates the pathogenesis of opportunistic infections in immunocompromised patients, specifically Toxoplasma gondii reactivation in AIDS. CD4+ T-cell depletion (<200 cells/mm³) predisposes to reactivation of latent infections, including toxoplasmosis, which presents with focal neurologic deficits and ocular involvement. **Why the Correct Answer is Right** Toxoplasma gondii causes reactivation encephalitis in AIDS patients with severe immunosuppression (CD4 <100 cells/mm³), but even counts <200 are risky. The patient’s high IgG indicates prior infection (common in populations with cat or undercooked meat exposure), now reactivated due to HIV-induced immunodeficiency. Retinochoroiditis and focal brain lesions are classic features. Living under a bridge (homelessness) suggests potential exposure to cat feces, the primary transmission route. **Why Each Wrong Option is Incorrect** **Option A:** Cryptococcus neoformans causes chronic meningitis with headache and fungal organisms in CSF, but lacks retinochoroiditis and focal brain lesions. **Option B:** Progressive multifocal leukoencephalopathy (PML) from JC virus presents with demyelination and at

✓ Correct Answer: C. Reactivation of bradyzoites in cysts from an earlier infection