**Core Concept**
Multiple sclerosis (MS) is a chronic autoimmune disease characterized by demyelination, inflammation, and axonal damage in the central nervous system (CNS). The disease primarily affects the white matter of the CNS, which consists of myelinated nerve fibers.

**Why the Correct Answer is Right**
In MS, the immune system attacks the myelin sheath surrounding the axons of neurons, leading to the formation of lesions. These lesions are areas of demyelination and axonal damage that can be visualized on magnetic resonance imaging (MRI). The white matter is particularly susceptible to damage due to its high concentration of myelinated nerve fibers. The lesions in MS typically appear as hyperintense areas on T2-weighted MRI images and as hypointense areas on T1-weighted images.

**Why Each Wrong Option is Incorrect**
* **Option B:** Grey matter is not the primary site of lesions in MS. While grey matter involvement can occur, it is less common and typically seen in later stages of the disease.
* **Option C:** The thalamus is a structure within the brain that can be affected by MS, but it is not a primary site of lesions. Lesions in the thalamus are often associated with specific clinical symptoms such as pain and sensory disturbances.
* **Option D:** The basal ganglia are a group of structures involved in movement control and motor function. While MS can affect the basal ganglia, it is not a primary site of lesions.

**Clinical Pearl / High-Yield Fact**
MS lesions are often described as "periventricular" because they occur near the ventricles, which are fluid-filled spaces within the brain. This location is characteristic of MS lesions and can be a helpful clue in diagnosing the disease.

✓ Correct Answer: A. White matter