**Core Concept**
The synovial fluid in joints is a thick, lubricating substance that reduces friction between the articular cartilage and underlying bone. It is composed of water, glycoproteins, and proteoglycans. The major proteoglycan in synovial fluid plays a crucial role in maintaining joint lubrication and viscosity.

**Why the Correct Answer is Right**
Aggrecan is the major proteoglycan of synovial fluid, responsible for its viscosity and lubricating properties. It consists of a protein core (aggreican) and multiple glycosaminoglycan (GAG) chains, primarily chondroitin sulfate and keratan sulfate. These GAG chains are negatively charged, allowing aggrecan to bind with hyaluronic acid, a large glycosaminoglycan in the synovial fluid, and form a network that maintains the fluid's viscosity and lubrication properties. This network is essential for reducing friction between the articular cartilage and underlying bone during joint movement.

**Why Each Wrong Option is Incorrect**
**Option A:** None of the other options are correct, as they do not accurately describe the major proteoglycan of synovial fluid.
**Option B:** Versican is a proteoglycan found in the extracellular matrix of various tissues, including cartilage and the synovial membrane, but it is not the major proteoglycan of synovial fluid.
**Option C:** Decorin is a small leucine-rich proteoglycan that regulates collagen fibrillogenesis and is found in various tissues, including skin and bone, but it is not the major proteoglycan of synovial fluid.

**Clinical Pearl / High-Yield Fact**
The composition of synovial fluid can be altered in various joint diseases, such as osteoarthritis, where the concentration of aggrecan and hyaluronic acid decreases, leading to reduced joint lubrication and increased friction.

**Correct Answer:** C. Aggrecan.