A 48-year-old woman presents with a long history of chronic constipation and straining with defecation despite augmented water intake, dietary, and escalating medical management. She relates that she strains to pass flatus. Colonoscopy was unremarkable. Radioopaque marker study (sitz marker) is found to have 22 out of 25 markers in the descending and sigmoid colon. Defecography demonstrates a decreased angulation of the colon with attempt at evacuation (nonrelaxation of the puborectalis) and with retained contrast in the vault after attempts at defecation. What is most likely to assist in the management of this patient?