A 15-year-old male presents with chronic low back pain. Lateral radiographs of the lumbar spine show anterior displacement of L5 over S1, with the posterior aspect of L5 covering 20% of the superior endplate of S1. According to Meyerding's classification, what grade of spondylolisthesis is this?

Explanation

Meyerding's classification grades spondylolisthesis based on the percentage of anterior displacement of one vertebral body over the one below it. The grades are as follows: * Grade I: 0-25% displacement * Grade II: 26-50% displacement * Grade III: 51-75% displacement * Grade IV: 76-100% displacement * Grade V (Spondyloptosis): >100% displacement In this case, 20% displacement falls into Grade I.