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?

Colorectal cancer is asymptomatic in its initial stages. As the tumor grows, the most common sign is occult blood in feces, especially when the tumor is in the proximal portion of the colon. Chronic, asymptomatic bleeding typically causes iron-deficiency anemia. Adenocarcinomas of the colon usually express CEA, a glycoprotein that is released into the circulation and serves as a serologic marker for these tumors. CEA is also found in association with malignant tumors of the pancreas, lung, and ovary. AFP (choice A) is expressed by hepatocellular carcinoma and yolk sac tumors. Chromogranin (choice D) is expressed by neuroendocrine tumors. Chorionic gonadotropin (choice C) is secreted by choriocarcinoma.Diagnosis: Colon cancer