**Core Concept**
S-100 protein is a family of acidic calcium-binding proteins, primarily found in the nervous system, but also expressed in melanocytes, chondrocytes, adipocytes, and macrophages. It plays a crucial role in the regulation of intracellular calcium levels and is involved in various cellular processes, including cell signaling and differentiation.

**Why the Correct Answer is Right**
S-100 protein is a highly specific marker for melanocytic tumors, including malignant melanoma. It is expressed in more than 90% of melanoma cells and is often used as a diagnostic marker in immunohistochemistry to distinguish melanoma from other types of skin cancers. The S-100 protein is particularly useful in identifying metastatic melanoma, as it is often positive in these cases.

**Why Each Wrong Option is Incorrect**
**Option A:** Cytokeratin is a marker for epithelial cells and is commonly used to identify carcinomas, not melanomas.
**Option B:** MBN-45 is not a recognized marker for malignant melanoma. It's possible that this option is a distractor or a misspelling.
**Option C:** Alpha FP (alpha-fetoprotein) is a tumor marker for certain types of cancer, including hepatocellular carcinoma and germ cell tumors, but it is not specific for malignant melanoma.

**Clinical Pearl / High-Yield Fact**
S-100 protein is also expressed in other types of tumors, including certain types of sarcomas and lymphomas, so it should be used in conjunction with other markers and clinical findings to establish a diagnosis.

**Correct Answer:**
✓ Correct Answer: D. S 100