A 42-year-old woman is on postoperative day 6 following open appendectomy for perforated appendicitis. She develops fever (39.2°C), tachycardia (110/min), and abdominal pain with localized tenderness and induration around the surgical incision. The wound appears intact with no drainage. Ultrasound shows a 4 cm hypoechoic collection adjacent to the incision. What is the most appropriate next step in management?