A 5-day-old female neonate born at 32 weeks gestation to a 22-year-old mother presents with feeding intolerance, abdominal distension, and bilious vomiting. The infant was on mechanical ventilation for 3 days for respiratory distress syndrome and was extubated 2 days ago. Temperature is 37.8°C, heart rate 165/min, respiratory rate 58/min, and blood pressure 55/35 mmHg. Abdomen is distended with absent bowel sounds. Plain abdominal X-ray shows pneumatosis intestinalis and portal venous gas. Blood culture is pending. What is the most likely diagnosis and the immediate next step?