In an upper trunk brachial plexus injury (Erb's palsy), which muscle is typically weak?

• A. Deltoid
• B. Finger intrinsics
• C. Triceps
• D. Upper trapezius

Answer: (A)

Erb's palsy occurs from injury to the upper trunk of the brachial plexus, most often affecting the C5–C6 nerve roots. Those roots feed the axillary nerve that innervates the deltoid, so when this pathway is disrupted the deltoid becomes weak and shoulder abduction is limited. That weakness of the deltoid is the hallmark of this pattern, leading to the typical presentation of the arm held close to the body with internal rotation. Finger intrinsics are controlled by lower roots (C8–T1) and are usually spared, the triceps is powered by the radial nerve (often C7–C8) and tends to be preserved, and the upper trapezius is mainly innervated by the spinal accessory nerve, so it is not selectively weakened in this injury.