Which of the following is NOT one of the four chambers of the heart?

The correct answer identifies "Lower atrium" as not being one of the four chambers of the heart. The heart is comprised of four main chambers: the right atrium, right ventricle, left atrium, and left ventricle. The atria (plural for atrium) serve as the upper chambers that receive blood, while the ventricles are the lower chambers that pump blood out of the heart.

Each of the recognized chambers plays a critical role in the circulatory process. The right atrium receives deoxygenated blood from the body, and the right ventricle pumps it to the lungs for oxygenation. The left atrium receives oxygenated blood from the lungs, and the left ventricle distributes it to the rest of the body.

The term "lower atrium" does not exist in cardiology and does not correctly reference any recognized part of the heart's structure, thus making it the correct answer for this question.