## Core Concept
Tumor markers are substances produced by cancer cells or in response to cancer. They can be used for diagnosing, monitoring, and predicting the outcome of various types of cancers. Common tumor markers include carcinoembryonic antigen (CEA), alpha-fetoprotein (AFP), human chorionic gonadotropin (hCG), and prostate-specific antigen (PSA).

## Why the Correct Answer is Right
The correct answer, **D. Creatinine**, is not recognized as a tumor marker. Creatinine is a waste product from the normal breakdown of muscle tissue, and its levels are commonly used to assess kidney function. Unlike the other options, which are associated with specific types of cancer, creatinine does not have a direct link to cancer diagnosis or monitoring.

## Why Each Wrong Option is Incorrect
- **Option A:** *Carcinoembryonic antigen (CEA)* is a recognized tumor marker, particularly for colorectal cancer, but also for other cancers like breast, lung, and pancreas.
- **Option B:** *Alpha-fetoprotein (AFP)* is a tumor marker primarily associated with hepatocellular carcinoma and certain germ cell tumors.
- **Option C:** *Human chorionic gonadotropin (hCG)* is a tumor marker for choriocarcinoma and certain germ cell tumors.

## Clinical Pearl / High-Yield Fact
A key point to remember is that while tumor markers can be helpful in the management of cancer patients, they are not specific or sensitive enough to be used alone for diagnosis. **Elevated levels of tumor markers can also be seen in benign conditions**, which can lead to false positives.

## Correct Answer Line
**Correct Answer: D. Creatinine.**