John is a 22-year-old professional skateboarder. He has recently fallen on his left shoulder which caused a severe dislocation of the joint. The clinician fears the impact and the dislocation may have caused damage to all or part of the brachial plexus. After performing a nerve conduction velocity test on Johnâs left arm, it was determined that the ulnar nerve was compromised. Which area of the brachial plexus was affected by the accident?