**Core Concept**
Viscosity of synovial fluid is a critical parameter that influences joint movement and lubrication. The viscosity of synovial fluid is primarily determined by the concentration of hyaluronic acid, which is a key component of the fluid. Hyaluronic acid is a high-molecular-weight polysaccharide that contributes to the fluid's lubricating properties.

**Why the Correct Answer is Right**
The correct answer, hyaluronic acid, plays a crucial role in maintaining the viscosity of synovial fluid. The high-molecular-weight polysaccharide chains of hyaluronic acid interact with each other, creating a network that resists shear stress and maintains the fluid's viscosity. This network also allows synovial fluid to maintain its lubricating properties, reducing friction between articular cartilage surfaces during joint movement. The concentration of hyaluronic acid in synovial fluid is a key determinant of its viscosity.

**Why Each Wrong Option is Incorrect**
**Option A:** Glycoproteins. Glycoproteins are present in synovial fluid but do not contribute significantly to its viscosity. They play a role in the fluid's immunological properties and interaction with cells, but their concentration does not determine the fluid's viscosity.

**Option B:** Serum proteins. Serum proteins, such as albumin and globulins, are present in synovial fluid but do not contribute to its viscosity. Their concentration in synovial fluid is relatively low, and they do not interact with the hyaluronic acid network to affect the fluid's viscosity.

**Option C:** Glucosamine and chondroitin sulfate. Glucosamine and chondroitin sulfate are both present in synovial fluid and play roles in cartilage metabolism and joint health. However, they do not contribute to the fluid's viscosity. Their concentration in synovial fluid is not a key determinant of the fluid's viscosity.

**Clinical Pearl / High-Yield Fact**
The viscosity of synovial fluid is a critical parameter that influences joint health and function. A decrease in synovial fluid viscosity, as seen in osteoarthritis, can lead to increased joint friction and inflammation. Conversely, an increase in synovial fluid viscosity, as seen in conditions such as pseudogout, can lead to joint swelling and pain.

**Correct Answer: A. Hyaluronic acid**