Which option best defines synovial fluid?

• A. Filtrate of blood flowing in the capillaries of the synovial membrane, which lubricates joints
• B. A fluid-filled bag cushioning tendons as they slide over bones and reduces friction
• C. A system that produces contractions to move the body, maintain posture, and move fluids through hollow tissues or organs
• D. Striated fibers attached to bone and responsible for movement at joints

Answer: (A)

Synovial fluid is the viscous liquid produced by the synovial membrane lining the joint capsule, formed as a filtrate of the capillary plasma. Its primary role is to lubricate the joint surfaces, allowing smooth articulation and reducing friction as bones glide over each other; it also supplies nutrients to avascular articular cartilage. The description fits because the fluid originates from blood plasma in the joint’s synovial membrane and serves as the joint lubricant. The other descriptions point to surrounding structures or tissues—such as a tendon sheath or bursa, or to the muscles and tendons themselves—rather than the lubricating fluid inside the joint.