What is the major vessel in the upper extremities that supplies blood to the arm?

The major vessel in the upper extremities that supplies blood to the arm is the brachial artery. This artery is a continuation of the axillary artery and runs down the upper arm, where it supplies blood to the muscles and tissues of the arm.

As it travels, the brachial artery gives off several branches and eventually bifurcates into the radial and ulnar arteries at the elbow. While the subclavian artery is significant as it supplies blood to the shoulder and upper limb, the brachial artery is specifically responsible for the direct blood supply to the entire upper arm. The radial and ulnar arteries, though important for supplying blood to the forearm and hand, branch off from the brachial artery and do not supply the arm in its entirety. Therefore, the brachial artery is the correct choice, as it is central to the vascular supply of the arm.