What is the tumor biomarker for colorectal cancer?

The tumor biomarker for colorectal cancer is Carcinoembryonic Antigen (CEA). CEA is a glycoprotein that is produced by certain types of cells in the body, including those found in the colon. Elevated levels of CEA can indicate the presence of colorectal cancer, and it is commonly used in clinical practice to monitor treatment response and detect recurrence after surgery. While it is not exclusively associated with colorectal cancer, as elevated CEA levels can also occur in other cancers and certain benign conditions, it is the most relevant and widely utilized biomarker specifically for this type of cancer.

The other options serve distinct roles in oncology but do not categorize as specific biomarkers for colorectal cancer. For example, CA-125 is primarily used as a biomarker for ovarian cancer. HER-2 is a marker associated with breast cancer, particularly in patients who have HER-2 positive tumors. AFP, or Alpha-fetoprotein, is typically linked to liver cancer and germ cell tumors. Therefore, while other tumor markers are crucial in their respective contexts, CEA stands out as the primary biomarker for colorectal cancer, making it the correct answer.