A 22-year-old woman presents with a 3-month history of abdominal distension and mild pain. Ultrasound reveals a large mixed echogenicity mass in the left ovary with cystic and solid components. Serum β-hCG is elevated at 45 mIU/mL, and AFP is 120 ng/mL. Which investigation is most appropriate to confirm the diagnosis and assess metastatic potential?