A 57-year-old woman presents to a physician with chronic pelvic pain of many years duration. On fuher questioning, the woman also repos urinary frequency, constipation, pain with intercourse and bloating. On physical examination, a large mass is felt in the pelvic area. Follow-up ultrasound examination demonstrates that the mass involves the right adnexa and is composed of multiloculated cystic spaces. Prior to sending the patient to surgery, the primary care physician wants to order a serum tumor marker to screen for ovarian cancer. Which of the following would be the best choice?