Which structure is NOT a pulmonary vein?

The middle cardiac structure is correctly identified as not being a pulmonary vein. Pulmonary veins are vessels that carry oxygenated blood from the lungs back to the heart. Specifically, in humans, there are four primary pulmonary veins: the right superior pulmonary vein, the right inferior pulmonary vein, the left superior pulmonary vein, and the left inferior pulmonary vein. Each of these arises from the respective lung and drains oxygen-rich blood into the left atrium of the heart.

In contrast, the middle cardiac vein is associated with the heart's venous system and is responsible for draining deoxygenated blood from the heart muscle itself. It does not participate in the oxygenation and transport of blood from the lungs but rather serves a different role in cardiac circulation. This distinction is critical in understanding the anatomy of the circulatory system and the roles various vessels play in blood circulation.