Blood is rich in oxygen in all of the following structures except for which?

The pulmonary artery is the correct choice because it is the only structure listed that carries deoxygenated blood. After blood picks up oxygen in the lungs, it returns to the heart through the pulmonary veins, which leads to the left atrium. From there, oxygen-rich blood is pumped into the left ventricle, which then sends it into the aorta for distribution throughout the body. Thus, the left ventricle, aorta, and left atrium all contain blood that is rich in oxygen, while the pulmonary artery carries blood that has just left the heart's right ventricle and is on its way to the lungs to receive oxygen, making it deoxygenated.