A 45-year-old woman with a history of chronic constipation is evaluated in the office. Recent transit study demonstrated retention of 16 Sitz markers scattered throughout the colon 5 days after ingesting the Sitzmark capsule. Colonoscopy findings are unremarkable. She has failed multiple medication trials. She is compliant with her dietary modifications such as fiber and fluid intake. She takes a commercial polyethylene glycol 3350 formulation as needed. What is the most appropriate next step in management?