A term infant with limb posture indicating injury after a difficult delivery most likely has injury to which structure?

• A. Brachial plexus
• B. Phrenic nerve
• C. Sternocleidomastoid muscle
• D. Radial nerve

Answer: (A)

Birth-related traction commonly injures the brachial plexus, especially the upper trunk (C5–C6). This leads to Erb-Duchenne palsy, with weakness of shoulder abduction and external rotation and elbow flexion. The result is the classic posture: the arm is adducted and internally rotated, with the forearm pronated and the elbow extended—the “waiter’s tip” position. This pattern reflects loss of function in nerves that supply the deltoid, supraspinatus, infraspinatus, and biceps.

The other options don’t fit this presentation. The phrenic nerve governs the diaphragm, so injury would cause breathing difficulties rather than a characteristic limb posture. A congenital sternocleidomastoid issue would cause neck tilt (torticollis) rather than a limb defect. Radial nerve injury would cause wrist drop, not the typical birth-associated arm posture.