What type of blood vessel always leaves an atrium?

The correct answer is: pulmonary arteries.

The pulmonary arteries are the only blood vessels that always leave an atrium. They carry deoxygenated blood from the right atrium to the lungs, where it is oxygenated. The pulmonary arteries are part of the pulmonary circulation, which is the circuit of blood from the heart to the lungs and back to the heart.