A 62-year-old man is evaluated for a 4- to 6-week history of passing bright red blood stool. He has no other symptoms. Medical history is unremarkable, and he takes no medications.

On physical examination, vital signs are normal. Abdominal examination is normal; the liver and spleen are not enlarged. Digital rectal examination reveals brown stool that is positive for occult blood.

Colonoscopy reveals a nonobstructing polypoid mass in the sigmoid colon. The remainder of the colon, from the ileocecal valve to the anus, is normal. Biopsy of the mass shows adenocarcinoma.

Which of the following diagnostic studies should be performed next?