A 45-year-old businesswoman arrives in your office with vague abdominal complaints. She has noticed melenic stool. Upon performing a sigmoidoscopy, you find a 4-cm mass in the upper colon. You should immediately order a blood test for which of the following tumor markers?

**Core Concept**
The question requires the identification of a tumor marker associated with a specific type of colorectal cancer. The underlying principle being tested is the correlation between tumor markers and their respective cancers. In this case, the correct tumor marker is associated with a malignancy of the epithelial cells lining the colon.

**Why the Correct Answer is Right**
The correct answer is associated with a tumor marker that is elevated in a specific type of colorectal cancer. This tumor marker is a glycoprotein that is produced by the tumor cells and can be detected in the blood. The correct tumor marker is specifically elevated in carcinomas of the colon, which are the most common type of colorectal cancer. The detection of this tumor marker in the blood can aid in the diagnosis and staging of the cancer.

**Why Each Wrong Option is Incorrect**
**Option A:** This tumor marker is associated with a different type of cancer, specifically pancreatic cancer. It is not the correct choice for this scenario.
**Option B:** This tumor marker is associated with a different type of cancer, specifically prostate cancer. It is not the correct choice for this scenario.
**Option D:** This tumor marker is associated with a different type of cancer, specifically hepatocellular carcinoma. It is not the correct choice for this scenario.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that tumor markers can aid in the diagnosis and staging of cancer, but they are not definitive diagnostic tools. A combination of clinical evaluation, imaging studies, and histopathological examination is necessary to confirm the diagnosis.

**Correct Answer:** C. CEA (Carcinoembryonic Antigen)

✓ Correct Answer: D. Carcinoembryonic antigen (CEA)