Acute Diverticulitis with Pericolic Abscess (Hinchey II)

A 62-year-old woman presents with 5 days of progressive left lower quadrant pain, fever (38.7°C), and a palpable tender mass. Laboratory studies show WBC 17,500 with left shift and CRP 218 mg/L. Contrast-enhanced CT of the abdomen and pelvis reveals sigmoid diverticulitis with segmental wall thickening, pericolic fat stranding, and a 5.2 cm rim-enhancing fluid collection (marked **B** in the diagram). There is no free intraperitoneal air. Which of the following is the most appropriate immediate management for the structure marked **B**?