A 52-year-old postmenopausal woman presents with progressive abdominal distension, early satiety, and pelvic pain for 3 months. On examination, a palpable pelvic mass is noted. Transvaginal ultrasound shows a complex ovarian mass with internal echoes and ascites. Serum CA-125 is elevated at 450 U/mL. What is the investigation of choice to confirm the diagnosis and stage the disease?