Which vessel is formed by the confluence of the right and left innominate veins?

The vessel that is formed by the confluence of the right and left innominate veins is the superior vena cava. Both the right and left innominate veins (also known as the brachiocephalic veins) collect blood from the head and upper limbs and then merge to form the superior vena cava, which is a major vein that carries deoxygenated blood from the upper body to the heart, specifically into the right atrium. This anatomical relationship is crucial for understanding venous drainage in the thoracic region.

The relevance of the other vessels noted in the options lies in their different roles within the circulatory system. The azygos vein provides alternative drainage from the thorax and drains into the superior vena cava, but it does not arise from the confluence of the innominate veins. The internal and external jugular veins are responsible for draining blood from the brain and face, and do not merge to form the superior vena cava. Thus, the correct choice highlights a fundamental aspect of venous anatomy and physiology pertinent to vascular studies.