Final answer:
An x-ray should visualize the epiphyseal plate to determine if a long bone is still growing. This plate is present in growing bones and is replaced by an epiphyseal line once growth ceases, typically in early adulthood.
Step-by-step explanation:
To determine if a long bone is still growing using an x-ray, the structure that should be visualized is the epiphyseal plate. As long bones develop, growth in length occurs at the epiphyseal plate, a layer of cartilage found at the metaphysis, where the diaphysis (shaft) meets the epiphysis (end) of the bone. In a growing bone, the epiphyseal plates are visible as darker areas on an X-ray because they are less dense than the osseous tissue. When the bone reaches maturity, typically in early adulthood (approximately 18-21 years), these plates are replaced by bone, forming an epiphyseal line. This transition from plate to line indicates that the bone has stopped growing in length.
Bones also grow in diameter through a process called appositional growth, which involves the medullary cavity and the periosteum. However, for assessing growth in length and determining if the bone is still growing, the epiphyseal plate is the key feature to examine on an x-ray.