A newborn presents with an adducted, internally rotated shoulder, extended elbow, and pronated forearm. This classic 'waiter's tip' deformity is most commonly associated with injury to which of the following nerve roots?

Explanation

Erb's palsy, or Erb-Duchenne palsy, typically involves the upper trunk of the brachial plexus, specifically the C5 and C6 nerve roots. This leads to weakness or paralysis of muscles supplied by these roots, including the deltoid, supraspinatus, infraspinatus, biceps, brachialis, and supinator. The resulting muscle imbalance causes the characteristic 'waiter's tip' posture: adducted and internally rotated shoulder (due to unopposed pectoralis major and latissimus dorsi), extended elbow (due to weak biceps and brachialis), and pronated forearm (due to weak supinator).