Which of the following muscle groups is primarily affected in a typical case of Erb's palsy, leading to the characteristic adducted and internally rotated shoulder posture?

Explanation

Erb's palsy affects the C5-C6 nerve roots. The deltoid (responsible for shoulder abduction), biceps (responsible for elbow flexion and forearm supination), and supraspinatus (initiates shoulder abduction) are key muscles innervated by these roots. Weakness in these muscles, combined with unopposed action of muscles like the pectoralis major and latissimus dorsi (internal rotators and adductors), leads to the classic adducted and internally rotated shoulder, extended elbow, and pronated forearm.