A 56-year-old otherwise healthy man underwent an elective laparoscopic sigmoid resection for diverticular disease 5 days ago has worsening abdominal pain, fever, and chills. On examination, his temperature is 102.2°F (39°C), pulse 120 beats/min, and blood pressure 85/58 mm Hg, and his abdomen is markedly tender in the bilateral lower quadrants with rebound and guarding. What is the best next step in management?