A 3-year-old boy from rural Maharashtra presents with a painless abdominal mass noticed by his mother during bathing. On examination, the mass is firm, non-tender, confined to one side of the abdomen, and does not cross the midline. Vital signs are normal. Hemoglobin is 10.2 g/dL. Abdominal ultrasound shows a large heterogeneous mass arising from the left kidney with no evidence of distant metastases on chest X-ray. What is the most appropriate next step in management?