**Core Concept**
S100 is a cytokeratin-like protein expressed in melanocytes, Schwann cells, and histiocytes. It is a valuable immunohistochemical marker for diagnosing tumors of neural and epithelial origin, particularly those arising from melanocytic or neural crest-derived cells.

**Why the Correct Answer is Right**
S100 protein is expressed in melanoma (due to melanocytic origin), schwannoma (neural sheath cells), and histiocytoma (histiocytic lineage). Basal cell carcinoma (BCC), although arising from the basal layer of the epidermis, does not express S100 protein consistently. Its diagnosis relies on other markers like p63 or CK14, and S100 is typically negative in BCC, making it the exception.

**Why Each Wrong Option is Incorrect**
Option A: Melanoma – S100 is strongly expressed in melanocytes, making it a key marker.
Option B: Schwannoma – Schwann cells express S100, which is a hallmark of this tumor.
Option C: Histiocytoma – Histiocytes are S100-positive, so this is a valid marker.

**Clinical Pearl / High-Yield Fact**
Always remember: S100 is **not** a reliable marker for basal cell carcinoma. If a lesion is S100-negative, it does not rule out BCC, but if it is S100-positive, it is more likely a melanoma or schwannoma. Use p63 or GATA3 for BCC diagnosis instead.

**✓ Correct Answer: D. Basal cell cancer**