The patient's age (12-year-old), activity (gymnast, involving repetitive hyperextension), and presentation (low back pain, anterior slip of L5 on S1) are classic for isthmic spondylolisthesis. This type is most commonly caused by a stress fracture or fatigue fracture of the pars interarticularis, often bilateral, leading to instability and anterior slippage. Degenerative disc disease and facet joint arthritis are more common in older adults. Congenital dysplastic facet joints can cause spondylolisthesis (dysplastic type), but a pars defect is more typical in an active adolescent.
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