What forms the superior vena cava?

The superior vena cava is formed by the confluence of the right and left brachiocephalic veins. Each brachiocephalic vein, in turn, is formed by the drainage of the head, neck, and upper extremities from the internal jugular and subclavian veins. This anatomical configuration allows the superior vena cava to efficiently return deoxygenated blood from the upper half of the body to the right atrium of the heart.

The other options involve veins that do not contribute to the formation of the superior vena cava. The jugular veins are responsible for draining blood from the head and neck but converge into the brachiocephalic veins, which are part of the correct answer. The internal and external iliac veins are associated with the lower body and do not participate in the formation of the superior vena cava, while subclavian veins primarily drain the arms; although they are part of the larger brachiocephalic vein system, they do not form the superior vena cava by themselves.