In lumbar spondylolisthesis, the defective portions of the vertebra are the

• A. Pars interarticularis
• B. Lamina
• C. Pedicle
• D. Vertebral body

Answer: (A)

Lumbar spondylolisthesis typically results from a bilateral stress fracture through the pars interarticularis, the narrow bridge of bone in the vertebral arch between the superior and inferior articular processes. When this portion is fractured on both sides, the posterior elements no longer provide firm stability, allowing the vertebral body to slip forward relative to the one below. The lamina, pedicle, and vertebral body are not the sites of this characteristic defect: the lamina forms the posterior arch, the pedicles connect the body to the arch, and defects in the vertebral body cause different injuries, not the classic forward slippage seen in spondylolisthesis. This pars region is a common site for stress injuries in athletes and is often evaluated with oblique imaging, where a pars fracture can be visualized and the resulting slip may be evident.