**Core Concept**
Endodermal sinus tumors, also known as yolk sac tumors, are a type of germ cell tumor that often secrete various hormones. The hormone produced by endodermal sinus tumors is key to understanding the pathophysiology and clinical presentation of this malignancy.

**Why the Correct Answer is Right**
Alpha-fetoprotein (AFP) is a major tumor marker produced by endodermal sinus tumors. It is a glycoprotein that plays a crucial role in fetal development, particularly in the formation of the yolk sac. In adults, elevated levels of AFP are often associated with germ cell tumors, including endodermal sinus tumors. The production of AFP by these tumors is thought to be due to the presence of immature fetal cells.

**Why Each Wrong Option is Incorrect**
* **Option A:** Human chorionic gonadotropin (hCG) is another hormone produced by germ cell tumors, particularly choriocarcinomas. However, it is not typically associated with endodermal sinus tumors.
* **Option B:** Estrogen is a hormone that can be produced by various types of tumors, including some germ cell tumors. However, it is not the primary hormone produced by endodermal sinus tumors.
* **Option C:** Progesterone is another hormone that can be produced by some tumors, but it is not typically associated with endodermal sinus tumors.

**Clinical Pearl / High-Yield Fact**
Elevated levels of AFP are a key diagnostic marker for endodermal sinus tumors, particularly in the context of germ cell tumors. This knowledge can aid in the early detection and management of this malignancy.