Necrotizing Pancreatitis with Walled-off Necrosis

A 45-year-old man with a history of alcohol abuse presents with severe epigastric pain, fever (38.5°C), and leukocytosis (WBC 14,500/μL) for the past 6 weeks. CT scan with contrast shows a large collection in the pancreatic region with a well-defined enhancing rim, as marked **B** in the diagram. The collection measures 8 cm and is causing persistent symptoms despite conservative management. According to the Revised Atlanta Classification 2012, what is the most appropriate FIRST-LINE intervention for this collection?