All of the following are found in the posterior triangle of the neck except one. Which one is the exception?

The hypoglossal (CN XII) nerve is not found in the posterior triangle; it is, however, present in the  submandibular triangle.  Contents of the posterior triangle include the external jugular and subclavian vein and their tributaries, the subclavian artery and its branches, branches of the cervical plexus, CN  XI, nerves to the upper limb and muscles of the triangle floor, the phrenic nerve, and the brachial plexus.