A motion segment comprises two vertebrae and a disc.

A motion segment comprises two vertebrae and a disc.

• A. Three vertebrae and a disc
• B. One vertebra and a disc
• C. Two vertebrae and two discs
• D. Two vertebrae and a disc

Answer: (D)

A motion segment is the smallest functional unit of the spine that allows movement. It is formed by two adjacent vertebrae and the intervertebral disc between them, with the surrounding facet joints and supporting ligaments stabilizing that pair. Between two neighboring vertebrae there is exactly one intervertebral disc, so the segment comprises two vertebrae and a disc. The other options would require either three vertebrae (two discs) or only one vertebra (no second vertebra to form a segment), or two discs between the same pair of vertebrae, which isn’t how the segment is arranged.

A motion segment is the smallest functional unit of the spine that allows movement. It is formed by two adjacent vertebrae and the intervertebral disc between them, with the surrounding facet joints and supporting ligaments stabilizing that pair. Between two neighboring vertebrae there is exactly one intervertebral disc, so the segment comprises two vertebrae and a disc. The other options would require either three vertebrae (two discs) or only one vertebra (no second vertebra to form a segment), or two discs between the same pair of vertebrae, which isn’t how the segment is arranged.