A 52-year-old man with chronic alcoholism presents with acute severe epigastric pain, fever (38.5°C), and tachycardia (HR 110/min). Laboratory values show lipase 2800 U/L, WBC 16,000/μL, and creatinine 1.8 mg/dL (baseline 0.9). Contrast-enhanced CT abdomen reveals extensive pancreatic necrosis involving >50% of the gland with peripancreatic fluid collection. Blood cultures are pending. What is the most appropriate next step in management?