An 18-year-old woman with a lifelong history of constipation requiring daily enema use presents to your office. Anorectal manometry cannot elicit a rectoanal inhibitory reflex. On contrast enema, the rectum is dilated down to the internal sphincter without a visible transition zone. Full-thickness biopsies taken of the rectum just proximal to the dentate line shows aganglionosis and hypertrophic nerve trunks. The biopsy specimen taken approximately 2 cm above the internal sphincter shows normal ganglion cells. What is the best next step in management?