A 82-year-old woman underwent a laparoscopic hand-assisted sigmoid colectomy with diverting loop ileostomy 5 days ago for a rectal cancer after neoadjuvant radiation therapy. She is tolerating a regular diet and her ileostomy is healthy and functional. The nurses report that she is somewhat somnolent, and has not urinated during the past two shifts. Which is the most likely cause of her anuria?