A 2-year-old boy is brought to the emergency department with a 3-week history of progressive irritability, poor feeding, and constipation. His mother reports he has been 'not himself' and has developed a limp over the past week. On examination, he is pale and has a firm, non-tender mass in the left upper abdomen that does not cross the midline. Vital signs show hypertension (BP 110/70 mmHg). Laboratory investigations reveal hemoglobin 8.5 g/dL, elevated urinary catecholamine metabolites (VMA and HVA), and a normal renal function. Abdominal ultrasound shows a left suprarenal heterogeneous mass with areas of calcification. Which of the following is the most likely diagnosis and the most appropriate initial imaging for staging?