**Core Concept**
Desmin is a type of intermediate filament protein that serves as a marker for muscle cell differentiation and is commonly expressed in muscle tumors, including rhabdomyosarcoma. This protein is crucial for the structural integrity and function of muscle cells, and its expression is often used as a diagnostic tool in pathology.

**Why the Correct Answer is Right**
Desmin is specifically expressed in rhabdomyosarcoma, a malignant tumor of skeletal muscle origin, due to its role in muscle cell differentiation. The presence of desmin in tumor cells is a strong indicator of their muscle lineage, making it an essential marker for diagnosing rhabdomyosarcoma. This is particularly useful in distinguishing rhabdomyosarcoma from other types of tumors, such as Ewing's sarcoma or embryonal rhabdomyosarcoma, which may also express other muscle-related proteins.

**Why Each Wrong Option is Incorrect**
**Option B:** Synaptophysin is a marker commonly used to diagnose neuroendocrine tumors, such as carcinoid tumors or small cell carcinoma of the lung, rather than muscle tumors like rhabdomyosarcoma.
**Option C:** Myeloperoxidase is a marker used to diagnose myeloid lineage cells, such as acute myeloid leukemia (AML), and is not relevant to the diagnosis of muscle tumors like rhabdomyosarcoma.
**Option D:** Cytokeratin is a marker used to diagnose epithelial lineage tumors, such as carcinomas, and is not typically expressed in muscle tumors like rhabdomyosarcoma.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that desmin is a sensitive marker for muscle tumors, including rhabdomyosarcoma, but its expression can be lost in poorly differentiated or dedifferentiated tumors. This can make diagnosis more challenging, and a combination of immunohistochemical markers may be required to confirm the diagnosis.

**✓ Correct Answer: A. Desmin**