A 42-year-old man from Delhi presents to the emergency department with severe epigastric pain radiating to the back for 6 hours, associated with vomiting. He has a history of alcohol abuse (40 units/week). On examination, he is febrile (38.5°C), tachycardic (110/min), and has severe epigastric tenderness with guarding. Serum amylase is 1200 U/L (normal <100), lipase 1800 U/L, and WBC 16,000/μL. Contrast-enhanced CT abdomen shows pancreatic edema with peripancreatic fluid collection and no necrosis. He is started on aggressive fluid resuscitation, NPO status, and broad-spectrum antibiotics. On day 5 of admission, despite optimal medical management, his clinical condition deteriorates: fever spikes to 39.2°C, abdominal pain worsens, and he develops signs of sepsis (lactate 4.2 mmol/L, BP 95/60). Repeat CT shows a 6 cm fluid collection with gas bubbles and rim enhancement. What is the most appropriate next step in management?