A 50-year-old woman, gravida 2, para 2, comes to your office reporting chronic constipation for 30 years. Her last colonoscopy was 6 months ago; findings showed a single 6-mm tubular adenoma, which was resected. She takes a daily polyethylene glycol 3350 (PEG) powder and psyllium husk and still only has two hard bowel movements a week. She admits to frequent manual disimpaction and weekly enemas. Magnetic resonance imaging (MRI) defecography demonstrated a 1.5-cm rectocele, which emptied with evacuation, and an anorectal manometry demonstrated paradoxical squeeze and incomplete sphincter relaxation consistent with obstructive defection. What is the best initial treatment option to improve her constipation?