A 28-year-old male labourer presents to the emergency department following a motorcycle accident with a severe traction injury to his left arm. On examination, he has loss of sensation over the lateral aspect of the forearm and dorsal aspect of the thumb and index finger. He also demonstrates weakness of shoulder abduction and external rotation. There is a characteristic 'waiter's tip' position of the left arm. Which nerve root(s) of the brachial plexus is/are most likely injured?