18 hours after the vaginal delivery of a newborn, a 22-year-old woman has weakness and numbness of her right foot. The delivery was complicated by prolonged labor and the patient had received epidural analgesia. Examination shows a high-stepping gait. There is weakness upon right foot dorsiflexion and right ankle eversion. The sensation is decreased over the dorsum of the right foot and the anterolateral aspect of the right lower extremity below the knee. Which of the following is the most likely cause of this patient's symptoms?