PLE-581

A 13-year-old girl suffers from a type of neural tube defect called tethered cord syndrome, a congenital anomaly that results from defective closure of the neural tube. This syndrome is characterized by an abnormally low conus medullaris, which is tethered by a short, thickened filum terminale, leading to progressive neurologic defects in the legs and feet. Which of the following defects is commonly associated with the tethered cord syndrome?