**Core Concept**
Endodermal sinus tumors (also known as yolk sac tumors) are a type of germ cell tumor that can occur in the ovaries, testes, or other locations in the body. These tumors are malignant and can secrete various substances, including tumor markers, which can aid in diagnosis.

**Why the Correct Answer is Right**
Alpha-fetoprotein (AFP) is a major tumor marker for endodermal sinus tumors. Elevated levels of AFP in the blood can indicate the presence of this type of tumor. The AFP protein is produced by the tumor cells and can be detected using a blood test. High levels of AFP are often associated with endodermal sinus tumors, particularly in the ovaries.

**Why Each Wrong Option is Incorrect**
**Option A:** Beta-hCG (human chorionic gonadotropin) is a tumor marker for choriocarcinoma, another type of germ cell tumor, but not for endodermal sinus tumors. Beta-hCG is produced by syncytiotrophoblast cells, which are not typically found in endodermal sinus tumors.
**Option B:** CA-125 is a tumor marker for various types of cancer, including ovarian cancer, but it is not specific for endodermal sinus tumors.
**Option C:** LDH (lactate dehydrogenase) is a non-specific enzyme that can be elevated in many types of cancer, including endodermal sinus tumors, but it is not a specific tumor marker for this tumor type.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that AFP levels can be elevated in other conditions, such as liver disease or pregnancy, so a high AFP level must be interpreted in the context of the patient's clinical presentation and other diagnostic findings.

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