**Core Concept**
Soft tissue sarcomas are a group of malignant tumors that arise from mesenchymal cells, which are the cells that give rise to connective tissue. They can occur in any part of the body, but are most commonly found in the extremities, trunk, and retroperitoneum.

**Why the Correct Answer is Right**
The most common variety of soft tissue sarcoma is the **leiomyyosarcoma**, which accounts for approximately 10-20% of all soft tissue sarcomas. Leiomyyosarcoma arises from the smooth muscle cells in the walls of blood vessels and hollow organs, and can occur in any part of the body. The pathogenesis of leiomyyosarcoma is not fully understood, but it is thought to involve genetic mutations that lead to uncontrolled cell growth and tumor formation.

**Why Each Wrong Option is Incorrect**
**Option A:** While **liposarcoma** is a common type of soft tissue sarcoma, it is not the most common variety.
**Option B:** **Rhabdomyosarcoma** is a type of sarcoma that arises from skeletal muscle cells, but it is relatively rare compared to leiomyyosarcoma.
**Option C:** **Synovial sarcoma** is a type of sarcoma that typically occurs near the joints of the arm, neck, or leg, but it is not the most common variety of soft tissue sarcoma.

**Clinical Pearl / High-Yield Fact**
It's worth noting that soft tissue sarcomas can be challenging to diagnose, and may be mistaken for benign tumors or other types of cancer. A high index of suspicion and a thorough diagnostic workup are essential for accurate diagnosis and treatment.

**Correct Answer:** A.