## 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 patients with a history of cancer.

## 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 other germ cell tumors. Prostate-specific antigen (PSA) is used as a marker for prostate cancer. Creatinine is a product of muscle metabolism that is excreted by the kidneys and is not specifically produced by cancer cells; its levels are used to assess kidney function.

## Why Each Wrong Option is Incorrect
* **Option A:** Alpha-fetoprotein (AFP) is a known tumor marker for hepatocellular carcinoma and certain germ cell tumors.
* **Option B:** Human chorionic gonadotropin (hCG) is a tumor marker for choriocarcinoma and other germ cell tumors.
* **Option C:** Prostate-specific antigen (PSA) is used as a tumor marker for prostate cancer.

## Clinical Pearl / High-Yield Fact
A key point to remember is that while tumor markers can aid in the diagnosis and monitoring of cancer, they are not specific or sensitive enough to be used alone for cancer screening in the general population. Creatinine, being a marker of kidney function rather than cancer, highlights the importance of understanding the specificities of different markers.

## Correct Answer: D. Creatinine.