Which of these structures is not a circulatory feature unique to the fetus? a. arterial duct b. oval opening c. umbilical vein d. pulmonary trunk

It's important to know that fetal circulation has specific unique features that differ from adult circulation. These features allow the fetus to receive oxygen and nutrients from the mother while bypassing certain organs (like the lungs) since they are not yet fully functional.

Let's analyze each of the given structures to see if they are unique to fetal circulation: a. Arterial duct - It is a blood vessel that connects the pulmonary artery to the aorta, bypassing the lungs. The arterial duct is present only in the fetus and usually closes shortly after birth. b. Oval opening - Also known as the foramen ovale, it is an opening in the atrial septum (the wall between the left and right atria) that allows blood to flow from the right atrium to the left atrium, bypassing the lungs. This structure usually closes shortly after birth. c. Umbilical vein - It carries oxygenated and nutrient-rich blood from the placenta to the fetus. It is a crucial part of the fetal circulation system and is no longer needed after birth, eventually becoming a fibrous cord called the round ligament of the liver. d. Pulmonary trunk - The pulmonary trunk is a major blood vessel that divides into the left and right pulmonary arteries. These arteries carry deoxygenated blood from the heart to the lungs in both the fetus and adult.

From our analysis of each option, we can notice that the pulmonary trunk (option d) is present and functions in both the fetus and adult, while the other options are unique to fetal circulation. So the correct answer is d. pulmonary trunk.