What structure lines the medullary cavity of a long bone?

The endosteum is the correct structure that lines the medullary cavity of a long bone. This thin membrane consists of a layer of osteoblasts and osteoclasts, which are essential for bone growth, repair, and remodeling. The endosteum plays a critical role in the maintenance of bone tissue and the processes that occur within the medullary cavity, including the production of bone marrow.

In contrast, cortical bone refers to the dense outer layer of bone that provides strength and support, which is different from the lining of the medullary cavity. The periosteum is a fibrous layer that covers the outer surface of bones and serves as an attachment point for tendons and ligaments; it does not line the medullary cavity. Articular cartilage is found at the ends of bones in joints and provides a smooth surface for movement, and it does not line the medullary cavity either. Understanding these distinctions helps clarify the specific functions and locations of various structures associated with long bones.