Which statement best describes synovial fluid?

• A. It is formed by plasma ultrafiltration
• B. It is derived from serum
• C. It lubricates the pneumocytes
• D. It is rich in hyaluronic acid

Answer: (A)

Synovial fluid is formed by ultrafiltration of plasma across the synovial membrane. As plasma filters through the membrane, it becomes a clear ultrafiltrate in the joint space, and the synovial lining further adds components like hyaluronic acid to give the fluid its viscous, lubricating properties. This makes it the best description, because it captures how the fluid originates. Although synovial fluid contains hyaluronic acid, and it lubricates joints, it is not derived from serum, and it does not lubricate pneumocytes (which are lung cells).