**Core Concept**
S100 is a family of calcium-binding proteins that are primarily expressed in cells of the immune system, particularly in dendritic cells and macrophages. These proteins play a crucial role in various cellular processes, including cell signaling and differentiation. The S100 proteins are also known to be involved in the pathogenesis of several autoimmune diseases.

**Why the Correct Answer is Right**
The correct answer is related to the role of S100 in the diagnosis and monitoring of multiple sclerosis (MS). S100B is a protein that is highly expressed in oligodendrocytes, the myelinating cells of the central nervous system. In MS, damage to these cells leads to the release of S100B into the cerebrospinal fluid (CSF), where it can be detected. This protein is often used as a biomarker for MS and can help differentiate it from other neurological conditions.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because S100 is not primarily associated with neurodegenerative diseases like Alzheimer's disease, although there may be some overlap in the pathophysiology.
**Option B:** This option is incorrect because while S100 is involved in immune responses, it is not a specific marker for rheumatoid arthritis.
**Option C:** This option is incorrect because S100 is not primarily associated with skin cancers, although some S100 proteins may be overexpressed in certain types of cancer.

**Clinical Pearl / High-Yield Fact**
It's worth noting that S100B levels can also be elevated in other neurological conditions, such as stroke and traumatic brain injury, so it's essential to consider the clinical context when interpreting S100B levels.

**Correct Answer: D. S100B is a marker of multiple sclerosis.**