**Core Concept**
Toxicoplasmic encephalitis (TE) and progressive multifocal leukoencephalopathy (PML) are common CNS lesions in HIV patients. These conditions are caused by opportunistic infections that take advantage of the compromised immune system.

**Why the Correct Answer is Right**
Toxicoplasmic encephalitis is caused by the protozoan Toxoplasma gondii, which reactivates in HIV patients with CD4 counts below 100 cells/μL. This leads to necrotizing encephalitis with perivascular inflammation and microglial nodules. PML, on the other hand, is caused by the JC virus, which infects oligodendrocytes and leads to demyelination and axonal loss.

**Why Each Wrong Option is Incorrect**
* **Option A:** Cryptococcal meningitis is another common CNS infection in HIV patients, but it is not the most common lesion. Cryptococcus neoformans causes meningitis by invading the subarachnoid space.
* **Option B:** Herpes simplex encephalitis is a viral infection that primarily affects immunocompetent individuals. While it can occur in HIV patients, it is not the most common CNS lesion.
* **Option C:** Cytomegalovirus (CMV) encephalitis is an opportunistic infection that can occur in HIV patients, but it is not as common as TE or PML.

**Clinical Pearl / High-Yield Fact**
In HIV patients, CNS lesions can be a presenting feature of immunosuppression. A high index of suspicion and prompt diagnosis are critical to initiate antiretroviral therapy and reduce mortality.

**Correct Answer: C. Progressive multifocal leukoencephalopathy.**