A 59-year-old man complains of progressive weakness. He reports that his stools are very dark. Physical examination demonstrates fullness in the right lower quadrant. Laboratory studies show iron deficiency anemia, with a serum hemoglobin level of 7.4 g/dL. Stool specimens are positive for occult blood. Colonoscopy discloses an ulcerating lesion of the cecum. Which of the following serum tumor markers is most likely to be useful for following this patient after surgery?