What is the joint-lubricating fluid secreted by the synovial membrane?

• A. Synovial fluid
• B. Cartilage
• C. Bone marrow
• D. Myelin

Answer: (A)

The fluid that lubricates joints is produced by the synovial membrane and fills the joint cavity as a slippery, viscous liquid. This synovial fluid creates a smooth film between the articular cartilage surfaces, greatly reducing friction during movement. It also provides nutrients to the avascular cartilage through diffusion and contains lubricating molecules like hyaluronic acid and lubricin that enhance slipperiness and help cushion loads.

Cartilage is the smooth tissue that lines joint surfaces, not a fluid. Bone marrow sits inside bones and makes blood cells, while myelin coats nerve fibers in the nervous system. None of these serve as the lubricating fluid of a joint.