**Core Concept**
The question describes a case of focal neurological deficit in an immunocompromised patient, with imaging and histopathological findings suggestive of a specific type of brain infection. The underlying concept here is the pathogenesis of opportunistic infections in immunocompromised hosts, particularly in the context of neurotoxoplasmosis.

**Why the Correct Answer is Right**
The correct answer is Toxoplasma gondii. This parasite is a common source of neurotoxoplasmosis, which typically presents as a ring-enhancing lesion in the brain, particularly in the white matter of the frontal lobe. The infection occurs when the parasite's cysts rupture, releasing bradyzoites that induce a chronic inflammatory response and coagulative necrosis of brain parenchyma. Toxoplasma gondii is a protozoan parasite that can infect any warm-blooded animal, including humans, and is transmitted through fecal-oral contact with contaminated cat feces.

**Why Each Wrong Option is Incorrect**
* **Option A:** Cryptococcus neoformans is another opportunistic pathogen that can cause CNS infections in immunocompromised hosts, but it typically presents as a mass lesion or meningitis, rather than a ring-enhancing lesion.
* **Option B:** Histoplasma capsulatum is a fungal pathogen that can cause CNS infections, but it is more commonly associated with meningitis or encephalitis, rather than a focal lesion like neurotoxoplasmosis.
* **Option D:** Nocardia species are aerobic actinomycetes that can cause CNS infections, but they typically present as abscesses or granulomas, rather than ring-enhancing lesions.

**Clinical Pearl / High-Yield Fact**
In patients with AIDS, neurotoxoplasmosis is a common cause of focal neurological deficits, particularly in those with CD4 counts <100 cells/μL. It is essential to consider this diagnosis in the context of characteristic imaging findings and histopathological features. **Correct Answer:** C. Toxoplasma gondii