Final answer:
A yolk sac tumor produces a hormone called alpha-fetoprotein (AFP).
Step-by-step explanation:
A yolk sac tumor, also known as an endodermal sinus tumor, is a type of germ cell tumor that typically occurs in the ovaries or testes. It is characterized by the production of a hormone called alpha-fetoprotein (AFP). AFP is normally produced by the yolk sac during fetal development, but when a yolk sac tumor forms, it produces excessive amounts of AFP, which can be detected in the blood. This elevated level of AFP is used as a diagnostic marker for yolk sac tumors.