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Carcinoembryonic antigen (CEA) is the most commonly elevated tumor marker in patients with suspected rectal cancer. CEA is a glycoprotein involved in cell adhesion, and it is produced during fetal development; however, its levels typically decrease after birth. In adults, elevated levels of CEA can indicate various types of cancers, particularly colorectal cancer, which includes rectal cancer.

Testing for CEA is particularly useful in the context of colorectal cancer because it can assist in the diagnosis, provide prognostic information, and monitor treatment response or detect recurrence after primary treatment. In cases of rectal cancer, elevated CEA levels often correlate with tumor burden and can be used in conjunction with imaging studies and other diagnostic methods to assess the disease's extent.

The other markers mentioned have different clinical applications. For instance, alpha-fetoprotein is primarily associated with liver cancer and germ cell tumors, while cancer antigen 125 is mainly utilized to track ovarian cancer. Carcinoma antigen 15-3 is primarily associated with breast cancer. Therefore, the elevation of CEA is specifically indicative of the pathological process associated with rectal and colorectal cancers, making it the most relevant choice in this context.