What structure forms the shaft of a long bone?

The diaphysis is the correct answer because it specifically refers to the elongated, tubular section that constitutes the shaft of a long bone. This central part of the bone is typically made of compact bone, which provides strength and support for weight-bearing activities. The diaphysis surrounds the medullary cavity, which contains bone marrow and plays a crucial role in the production of blood cells.

In contrast, the epiphysis refers to the end part of the long bone, which is involved in the articulation with adjacent bones at a joint. The growth plate, or epiphyseal plate, is found at the junction between the epiphysis and diaphysis and is responsible for bone elongation during development. Lastly, the medullary cavity is the central space within the diaphysis itself, housing the bone marrow rather than being a structural component of the bone's shaft. Understanding these terms helps clarify the anatomy of long bones and their functions in the skeletal system.