A Baker's cyst is primarily a collection of what substance?

A Baker's cyst is primarily a collection of synovial fluid. This fluid is a slippery substance produced by the synovial membrane, which lines joints and provides lubrication to facilitate smooth movement. When there is inflammation or excess production of synovial fluid—often due to conditions such as arthritis or a knee injury—this excess fluid can become trapped in a sac that forms behind the knee, resulting in a Baker's cyst.

The presence of synovial fluid in a Baker's cyst is important as it reflects the underlying joint pathology that often contributes to the formation of the cyst, rather than blood, serous fluid, or joint cartilage. Blood would typically indicate hemorrhage, while serous fluid, while similar, is not the main constituent associated with a Baker's cyst. Joint cartilage is a structural component of the knee, but it is not a fluid and does not accumulate in a cyst. Thus, synovial fluid is the key substance that characterizes a Baker's cyst.