A 2-year-old girl from Delhi presents with a 2-month history of progressive abdominal distension and irritability. Her mother reports she has been 'paler than usual' over the past month. On examination, there is a firm, non-tender mass in the left upper quadrant extending across the midline. Hemoglobin is 8.9 g/dL. Abdominal ultrasound reveals a large heterogeneous suprarenal mass with areas of calcification. Urine catecholamine metabolites (VMA and HVA) are markedly elevated. Chest X-ray shows no pulmonary lesions. What is the most appropriate next step in management?