If an accident victim had a severed left subclavian artery, which pulse (neck or wrist) would be most affected? why?

The wrist.: Blood flow in arteries is away from the heart. The subclavian artery goes into the arm and is past where the vessels to the neck are located. If it is cut then flow to the forearm and wrist may be less.

Anantomy test ?: If you hop on the internet you can find the SCA anatomy and follow the SCA to the brachial and then the radial and ulnar and you'll have your answer. But in reality , anyone who severs that artery has a good chance of bleeding out and would probably die if not treated immediately ...., and in that case ALL of your arteries pulses would be 'affected'