A 52-year-old woman undergoes a sigmoid resection with primary anastomosis for recurrent diveiculitis. She returns to the emergency room 10 days later with left flank pain and decreased urine output; laboratory examination is significant for a white blood cell (WBC) count of 20,000/mm3 . She undergoes a CT scan that demonstrates new left hydronephrosis, but no evidence of an intraabdominal abscess. Which of the following is the most appropriate next step in management?