**Core Concept**
Endodermal sinus tumors, also known as yolk sac tumors, are a type of germ cell tumor that primarily affects the gonads but can also occur in extragonadal sites. These tumors are characterized by the production of specific tumor markers, which aid in their diagnosis.

**Why the Correct Answer is Right**
Alpha-fetoprotein (AFP) is a major tumor marker produced by endodermal sinus tumors. Elevated levels of AFP in the serum or cerebrospinal fluid are highly suggestive of this tumor type. The production of AFP is thought to be related to the embryonic development of the yolk sac. The tumor cells in endodermal sinus tumors express AFP, which can be detected using immunohistochemical staining.

**Why Each Wrong Option is Incorrect**
* **Option A:** Human chorionic gonadotropin (hCG) is a tumor marker associated with choriocarcinoma, another type of germ cell tumor, but not with endodermal sinus tumors.
* **Option B:** Lactate dehydrogenase (LDH) is a nonspecific marker that can be elevated in various types of tumors, including germ cell tumors, but it is not specific for endodermal sinus tumors.
* **Option D:** Beta-hCG is a variant of hCG, and while it can be elevated in certain types of tumors, it is not a primary marker for endodermal sinus tumors.

**Clinical Pearl / High-Yield Fact**
Endodermal sinus tumors are highly sensitive to chemotherapy and radiation therapy, and early detection through tumor marker screening can improve patient outcomes.

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