**Core Concept**
Multiple sclerosis (MS) is a chronic autoimmune disease characterized by demyelination, inflammation, and axonal damage in the central nervous system (CNS). Magnetic resonance imaging (MRI) is a crucial diagnostic tool for detecting lesions in MS patients.

**Why the Correct Answer is Right**
The correct answer is related to the pathophysiology of MS. MS lesions typically occur in the white matter of the CNS, where myelinated nerve fibers are most concentrated. These lesions result from the damage caused by autoreactive T cells and macrophages that target the myelin sheath, leading to demyelination and axonal degeneration. The most common locations for MS lesions are the periventricular regions, the corpus callosum, and the posterior fossa.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because MS lesions are not typically found in the grey matter of the CNS. Grey matter is primarily composed of neuronal cell bodies and is less susceptible to demyelination.
* **Option B:** This option is incorrect because MS lesions are not typically found in the peripheral nervous system (PNS). The PNS is composed of nerve fibers that are not myelinated or are only partially myelinated, making it less susceptible to demyelination.
* **Option C:** This option is incorrect because MS lesions are not typically found in the cerebrospinal fluid (CSF). While CSF analysis may reveal oligoclonal bands or elevated protein levels in MS patients, the lesions themselves are located in the CNS tissue.

**Clinical Pearl / High-Yield Fact**
When interpreting MR images in MS patients, it's essential to look for lesions in the periventricular regions, corpus callosum, and posterior fossa. These locations are most commonly affected in MS, and detecting lesions in these areas can help support a diagnosis of the disease.

**Correct Answer:** C. White matter.