A 51-year-old immunosuppressed patient with a medical history significant for kidney transplant and chronic obstructive pulmonary disease presents to the emergency department with left lower quadrant abdominal pain. Computed tomography (CT) of the abdomen and pelvis demonstrates sigmoid diverticulitis with free fluid and spicules of free air in the pelvis. The patient is tachycardic, febrile, and diaphoretic, with lower quadrant abdominal tenderness. You decide to operate. What is the best next step in management?