**Core Concept**
Tumor markers are substances produced by cancer cells or in response to cancer. They can be proteins, enzymes, or other molecules that help diagnose, monitor, or predict the progression of cancer.

**Why the Correct Answer is Right**
Tumor markers are used to detect cancer, particularly in its early stages. They can also be used to monitor the effectiveness of treatment and detect recurrence. Common tumor markers include prostate-specific antigen (PSA), carcinoembryonic antigen (CEA), and alpha-fetoprotein (AFP).

**Why Each Wrong Option is Incorrect**
- **Option A:** This is a tumor marker, often elevated in patients with prostate cancer.
- **Option B:** This is a tumor marker, commonly used to monitor ovarian cancer.
- **Option C:** This is a tumor marker, typically elevated in patients with colorectal cancer.

**Clinical Pearl / High-Yield Fact**
Some tumor markers, such as PSA and CEA, can be elevated in non-cancerous conditions, which can lead to false-positive results.

**Correct Answer: D.**