**Core Concept:** Synovial sarcoma is a rare malignant tumor that arises from mesenchymal cells, particularly in the extremities and the trunk. It is characterized by chromosomal translocation t(X;18)(p11.2;q11.2), which results in the fusion of SYT and SSX genes. This translocation leads to the production of a fusion protein that plays a crucial role in the development and progression of the tumor.

**Why the Correct Answer is Right:** The correct answer is D, which states "SS18-SYT." This is because the fusion protein formed due to the chromosomal translocation t(X;18)(p11.2;q11.2) is SS18-SYT. The SS18 (Syt-1) gene is located on chromosome 18, while the SYT (SYT) gene is located on chromosome X. The fusion of these two genes results in the production of SS18-SYT fusion protein, which is a specific marker for synovial sarcoma.

**Why Each Wrong Option is Incorrect:**

A. "SSX-SYT" is incorrect as the correct fusion protein is SS18-SYT, not SSX-SYT.

B. "SSX-SSX" is incorrect because the fusion protein is SS18-SYT, not SSX-SSX.

C. Osteosarcoma is another type of malignant bone tumor, but the question specifically asks for the marker of synovial sarcoma, which is SS18-SYT.

**Clinical Pearl:** Understanding the specific marker for a disease, such as SS18-SYT in the case of synovial sarcoma, is crucial for accurate diagnosis, prognosis, and targeted therapies. Molecular markers like SS18-SYT can help differentiate synovial sarcoma from other similar tumors and guide clinicians in making informed decisions regarding treatment strategies.