A 77-year-old man has recently undergone a low anterior resection with diverting loop ileostomy for rectal cancer after total neoadjuvant therapy. His hospital course was routine, and he was sent home on postoperative day 4. The patient presents back to the clinic 3 weeks after the index operation. He reports urinary retention, pelvic pain, and bloating. You note an overall failure to thrive. Which test is the most likely to correctly diagnose this patient’s problem?