## **Core Concept**
Tumor markers are substances produced by cancer cells or in response to cancer. They can be found in higher-than-normal amounts in the blood, urine, or tissues of some patients with certain types of cancer. Tumor markers are used for monitoring response to treatment, detecting recurrence, and sometimes for screening.

## **Why the Correct Answer is Right**
Alpha-fetoprotein (AFP) is a tumor marker associated with hepatocellular carcinoma and certain germ cell tumors. Human chorionic gonadotropin (hCG) is a marker for choriocarcinoma and some germ cell tumors. Prostate-specific antigen (PSA) is used primarily as a marker for prostate cancer. Creatinine is a product of muscle metabolism that is excreted by the kidneys and is used as a marker of kidney function, not as a tumor marker.

## **Why Each Wrong Option is Incorrect**
* **Option A:** AFP is a known tumor marker for hepatocellular carcinoma and certain germ cell tumors.
* **Option B:** hCG is a tumor marker for choriocarcinoma and some germ cell tumors.
* **Option D:** PSA is a tumor marker for prostate cancer.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that while tumor markers can be helpful in managing cancer patients, they are not specific or sensitive enough to be used for screening the general population for cancer. Elevated levels of a tumor marker can also be seen in non-cancerous conditions.

## **Correct Answer:** C. Creatinine.