Where is the lowest blood oxygen saturation usually found in the body?

The lowest blood oxygen saturation in the body is typically found in the coronary sinus. The coronary sinus is responsible for collecting deoxygenated blood from the myocardium (the heart muscle) and draining it into the right atrium. Since this blood has just returned from delivering oxygen to the heart tissue, it has a relatively low oxygen saturation compared to blood in other parts of the circulatory system where oxygenation has occurred, such as the pulmonary veins or the left ventricle.

In contrast, the right atrium does receive deoxygenated blood from the body but also has contributions from the azygos venous system, which can slightly elevate saturation levels compared to that found in the coronary sinus. The left ventricle contains oxygenated blood that has just come from the lungs, thus displaying a higher saturation level. The pulmonary artery carries deoxygenated blood from the right ventricle to the lungs for oxygenation and therefore exhibits lower saturation than the systemic circulation but higher than that found in the coronary sinus.

Thus, among the choices provided, the coronary sinus is where the lowest blood oxygen saturation is found, reflecting the nature of blood returning from the heart's own tissues with the least amount of available oxygen.