Primary cause of spondylolisthesis?

• A. Disc degeneration
• B. Pars fracture
• C. Ligament laxity
• D. Facet hypertrophy

Answer: (B)

Spondylolisthesis most often starts from a defect in the pars interarticularis. When this structure sustains a bilateral stress fracture, the posterior elements no longer provide enough resistance to forward shear, so the vertebral body can slip anteriorly relative to the one below. This paras fracture essentially removes the stabilizing bridge between the vertebrae, making slippage the natural progression, especially at the lumbosacral junction with repetitive extension and loading seen in young athletes.

Disc degeneration, ligament laxity, and facet hypertrophy can contribute to instability or alter mechanics, but they are not the primary initiating lesion that causes the slip; the pars fracture is the classic initiating cause.