**Core Concept**
Prostate-specific antigen (PSA) is a serine protease enzyme produced by the prostate gland. It plays a crucial role in the liquefaction of semen. Elevated levels of PSA in the blood can indicate prostate cancer, prostatitis, or benign prostatic hyperplasia (BPH).

**Why the Correct Answer is Right**
PSA is used as a tumor marker for prostate cancer due to its ability to be secreted and detected in the blood. The enzyme's structure and function are closely related to its potential as a biomarker for prostate cancer. PSA levels can be used to monitor the progression of prostate cancer and to evaluate the effectiveness of treatment.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because PSA is not a diagnostic marker for other cancers, although it can be elevated in various conditions, including prostatitis and BPH.
**Option B:** This option is incorrect because hCG (human chorionic gonadotropin) is a tumor marker for testicular cancer, not prostate cancer.
**Option C:** This option is incorrect because AFP (alpha-fetoprotein) is a tumor marker for hepatocellular carcinoma and certain germ cell tumors, not prostate cancer.

**Clinical Pearl / High-Yield Fact**
PSA levels can be artificially elevated by digital rectal examination (DRE) and certain medical procedures, such as prostate biopsy. Therefore, it is essential to interpret PSA levels in the context of the patient's clinical history and other diagnostic findings.

**Correct Answer: D. Prostate cancer screening.**