The blood vessel which discharges into the right atrium (auricle) is the:

The blood vessel that discharges into the right atrium is the inferior vena cava. This large vein is responsible for carrying deoxygenated blood from the lower half of the body back to the heart. It collects blood from the lower body and returns it to the right atrium, where it enters the heart for pulmonary circulation.

The superior vena cava, while also discharging into the right atrium, brings blood from the upper body. The pulmonary veins, on the other hand, carry oxygenated blood from the lungs to the left atrium, and the coronary sinus drains blood from the heart's own myocardium back into the right atrium. Hence, while the right atrium does receive blood from several sources, the inferior vena cava specifically serves to deliver the deoxygenated blood from the lower body directly into the heart.