**Core Concept**
Progressive multifocal leukoencephalopathy (PML) is a rare and often fatal demyelinating disease of the central nervous system caused by the reactivation of the JC virus, a polyomavirus that targets oligodendrocytes and leads to demyelination.

**Why the Correct Answer is Right**
PML is characterized by the progressive damage to white matter in the brain, leading to symptoms such as weakness, vision loss, and cognitive decline. The JC virus infects oligodendrocytes, which are responsible for myelinating axons in the central nervous system. The virus disrupts the normal function of these cells, resulting in the demyelination characteristic of PML. The disease typically affects individuals with weakened immune systems, such as those with HIV/AIDS or taking immunosuppressive medications.

**Why Each Wrong Option is Incorrect**
**Option A:** JC virus is indeed the causative agent of PML, making this statement true and not the correct answer.
**Option B:** While PML does affect the brain, it primarily involves white matter, not cortical gray matter, making this statement incorrect.
**Option C:** PML is indeed a disease of white matter, as it involves the demyelination of oligodendrocytes in the central nervous system, making this statement true and not the correct answer.
**Option D:** PML does not primarily affect gray matter, which is composed of neurons and other cell types, but rather the white matter, which is composed of myelinated axons and oligodendrocytes.

**Clinical Pearl / High-Yield Fact**
PML is often associated with immunosuppression, particularly in patients with HIV/AIDS or those taking immunosuppressive medications. The disease is characterized by a distinctive radiographic pattern on MRI, with lesions that are typically located in the subcortical white matter.

**Correct Answer:** D. Disease of grey matter