Final answer:
The posterior interosseous nerve can be traced back to the radial nerve during a posterior approach to the humerus.
Step-by-step explanation:
The nerve that can be used to trace back to the radial nerve during a posterior approach to the humerus is the posterior interosseous nerve. This nerve is a branch of the radial nerve and enters the posterior compartment of the forearm by passing between the two heads of the supinator muscle. It innervates the deep posterior compartment muscles of the forearm.