**Core Concept**
Testicular yolk sac tumors are a type of germ cell tumor, which are the most common type of testicular cancer in young men. These tumors secrete various tumor markers, which can be used for diagnosis and monitoring of the disease.

**Why the Correct Answer is Right**
Alpha-fetoprotein (AFP) is a major tumor marker for testicular yolk sac tumors. Elevated levels of AFP in the serum can be a diagnostic indicator of this type of tumor. AFP is a glycoprotein produced by the yolk sac endoderm during fetal development, and its production is reactivated in yolk sac tumors. The mechanism behind AFP production in yolk sac tumors involves the expression of AFP genes by tumor cells.

**Why Each Wrong Option is Incorrect**
**Option A:** Human chorionic gonadotropin (hCG) is a tumor marker for choriocarcinoma, a different type of germ cell tumor. Elevated hCG levels are not typically associated with yolk sac tumors.
**Option B:** Lactate dehydrogenase (LDH) is a nonspecific marker that can be elevated in various types of cancer, including testicular cancer. However, it is not specific to yolk sac tumors.
**Option C:** Prostate-specific antigen (PSA) is a marker for prostate cancer, which is not related to testicular yolk sac tumors.

**Clinical Pearl / High-Yield Fact**
Testicular yolk sac tumors often present with elevated AFP levels, which can be used as a diagnostic marker. It is essential to note that AFP levels can be elevated in other conditions, such as liver disease, so a combination of clinical and laboratory findings is necessary for accurate diagnosis.

**Correct Answer:** C. Alpha-fetoprotein (AFP)