What runs vertically downward on the posterior aspect of the anterior thoracic wall?

The internal thoracic artery runs vertically downward on the posterior aspect of the anterior thoracic wall. This artery originates from the subclavian artery and descends along the inside of the rib cage, providing blood supply to the anterior chest wall and the breasts. It lies directly alongside the sternum and is an important vessel for supplying the diaphragm, intercostal muscles, and other structures in the thoracic cavity.

In this context, the other vessels mentioned serve different functions and locations. The subclavian artery is located further upstream and mainly supplies the upper limbs and neck regions before it becomes the axillary artery. The aorta, while a major artery, passes down through the thorax more posteriorly compared to the anterior thoracic wall and primarily supplies blood to the body after it leaves the heart. The brachial artery is found within the arm and is a continuation of the axillary artery, thus not related to the posterior aspect of the anterior thoracic wall. Therefore, the internal thoracic artery is the most appropriate answer for this question.