A man with a history of fall presented with a deformity which is resembling 'a waiter who waits for receiving his tip'. This posture is characteristically seen in?

The upper trunk (C5 and C6) of brachial plexus is the most common area injured and results in the classic Erb palsy. It has been described as the "waiter's tip posture" and is characterized by shoulder weakness with internal rotation and adduction of the upper arm. The elbow is extended and the wrist flexed. There is good preservation of hand function.