Which structure provides nutrients to the embryo and is the initial site of AFP?

• A. Yolk sac
• B. Amnion
• C. Chorion
• D. Placenta

Answer: (A)

In early development the yolk sac is responsible for nourishing the embryo before the placenta is fully established to handle nutrient transfer. It also serves as the first source of alpha-fetoprotein (AFP) produced by the developing fetus; AFP production by the yolk sac is prominent in the first trimester and later shifts toward the fetal liver. The other structures have different roles: the amnion forms the protective amniotic sac around the embryo, the chorion contributes to placental formation, and the placenta becomes the main site of nutrient and gas exchange as gestation progresses. Because the yolk sac provides nutrients early and is the initial producer of AFP, it best fits both aspects of the question.