Which vessel drains the upper part of the thoracic wall on the left side?

The accessory hemiazygos vein is responsible for draining the upper part of the thoracic wall on the left side. This vein runs alongside the vertebral column and collects blood from the left posterior intercostal veins from the 5th to the 8th intercostal spaces. It typically joins the azygos vein at a level near the 7th thoracic vertebra.

The azygos vein primarily drains the right side of the thoracic wall and the thoracic cavity. Meanwhile, the inferior vena cava collects deoxygenated blood from the lower body and does not drain the thoracic structures. The internal thoracic vein drains blood from the anterior chest wall and provides venous return from the medial thoracic region but is not specifically associated with draining the upper thoracic wall on the left side. Therefore, the accessory hemiazygos vein is the correct vessel for this particular drainage task.