**Core Concept**
Immunohistochemistry is a diagnostic tool that utilizes antibodies to detect specific antigens in tissue samples. HMB-45 is a monoclonal antibody that targets an antigen present in melanocytic cells, making it a valuable marker for diagnosing melanoma and other melanocytic lesions.

**Why the Correct Answer is Right**
HMB-45 reacts with an antigen present in the melanosomal protein gp100, which is a component of the melanosome, the organelle responsible for melanin production in melanocytes. The gp100 protein is specifically expressed in melanocytic cells, including melanoma cells. This makes HMB-45 a sensitive and specific marker for diagnosing melanoma and other melanocytic malignancies.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because HMB-45 is not typically used as a marker for lymphoid malignancies. The markers used to diagnose lymphomas include CD markers, such as CD20 and CD30.

**Option B:** This option is incorrect because HMB-45 is not typically used as a marker for epithelial malignancies. The markers used to diagnose epithelial malignancies include cytokeratins, such as CK7 and CK20.

**Option C:** This option is incorrect because HMB-45 is not typically used as a marker for hematopoietic malignancies. The markers used to diagnose hematopoietic malignancies include CD markers, such as CD45 and CD33.

**Option D:** This option is incorrect because HMB-45 is not typically used as a marker for neural malignancies. The markers used to diagnose neural malignancies include neurofilament proteins and S-100 protein.

**Clinical Pearl / High-Yield Fact**
When interpreting immunohistochemistry results, it's essential to consider the specific antibody used and the tumor type being diagnosed. HMB-45 is a valuable marker for diagnosing melanoma, but it can also be positive in other melanocytic lesions, such as spindle cell melanoma and blue nevus.

**Correct Answer: C. Melanoma**