Final answer:
The tumor marker used in colorectal cancer to follow the course of the disease is CEA (carcinoembryonic antigen).
Step-by-step explanation:
In the case of colorectal cancer, the tumor marker used to follow the course of the disease is CEA (carcinoembryonic antigen).
CEA is a blood-borne protein that is produced by tumors of the gastrointestinal system and occasionally by lung and breast cancer. While an elevated level of CEA does not specifically indicate bowel cancer, it can be a sign of bowel cancer recurrence in patients with a history of treated bowel cancer. Monitoring CEA levels can provide reassurance to post-operative patients and indicate the need for further diagnostic tests.