**Core Concept**
The question is testing the student's knowledge of tumor markers used for follow-up in colorectal cancer (Ca colon). Carcinoembryonic antigen (CEA) is a protein that is often elevated in the blood of patients with colorectal cancer.

**Why the Correct Answer is Right**
CEA is a tumor marker that is commonly used for follow-up in patients with colorectal cancer. Elevated levels of CEA in the blood can indicate recurrence or metastasis of the disease. CEA is produced by epithelial cells, and its levels are often elevated in patients with colorectal cancer due to the tumor's ability to produce and secrete this protein. This makes CEA a useful marker for monitoring the response to treatment and detecting recurrence in patients with colorectal cancer.

**Why Each Wrong Option is Incorrect**
**Option B:** CA 125 is a tumor marker that is primarily used for monitoring ovarian cancer, not colorectal cancer. Elevated levels of CA 125 in the blood can indicate ovarian cancer, but it is not a useful marker for follow-up in patients with colorectal cancer.
**Option C:** CA 19-9 is a tumor marker that is primarily used for monitoring pancreatic cancer, not colorectal cancer. Elevated levels of CA 19-9 in the blood can indicate pancreatic cancer, but it is not a useful marker for follow-up in patients with colorectal cancer.
**Option D:** AFP (alpha-fetoprotein) is a tumor marker that is primarily used for monitoring hepatocellular carcinoma and certain germ cell tumors, not colorectal cancer. Elevated levels of AFP in the blood can indicate these types of cancer, but it is not a useful marker for follow-up in patients with colorectal cancer.

**Clinical Pearl / High-Yield Fact**
It's essential to note that CEA levels can be elevated in other conditions, such as smoking, pancreatitis, and certain types of cancer, so it should be used in conjunction with other diagnostic tests and clinical evaluation.

**✓ Correct Answer: A. CEA**