## **Core Concept**
The patient's presentation suggests a diagnosis of colorectal cancer, specifically an ulcerating lesion in the cecum, which is a common site for this type of tumor. The management and follow-up of colorectal cancer often involve monitoring for recurrence and metastasis using serum tumor markers.

## **Why the Correct Answer is Right**
The correct answer, **C. Carcinoembryonic antigen (CEA)**, is a tumor marker commonly used for monitoring patients with colorectal cancer after surgery. CEA is a protein that can be elevated in various types of cancers, including colorectal, breast, lung, and pancreatic cancers. In the context of colorectal cancer, CEA levels are often measured before and after treatment to assess response to therapy and to monitor for recurrence. A decrease in CEA levels after surgery indicates a good response to treatment, while rising levels may suggest recurrence or metastasis.

## **Why Each Wrong Option is Incorrect**
- **Option A: Alpha-fetoprotein (AFP)** is primarily used as a tumor marker for hepatocellular carcinoma and certain types of germ cell tumors, not colorectal cancer.
- **Option B: Human chorionic gonadotropin (hCG)** is elevated in choriocarcinoma, certain germ cell tumors, and some other conditions, but it is not typically used for monitoring colorectal cancer.
- **Option D: Prostate-specific antigen (PSA)** is a specific marker for prostate cancer and is not relevant to the follow-up of colorectal cancer.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that while CEA is useful for monitoring recurrence in colorectal cancer, it is not diagnostic on its own due to its lack of specificity and sensitivity. It should be used in conjunction with imaging studies and clinical assessment. Additionally, not all patients with colorectal cancer will have elevated CEA levels, making regular monitoring and other diagnostic methods essential.

## **Correct Answer: C. Carcinoembryonic antigen (CEA).**