A 22-year-old pregnant woman was admitted emergently to the hospital after the baby had begun to appear at the introitus. The baby had presented in the breech position, and it had been necessary to exert considerable traction to complete the delivery and the newborn is shown. Which of the following structures was most likely injured by the trauma of childbirth?

(b) Source: GAS 700-709; GA 361, 369-371During a breech delivery as described here, downward traction is applied to the shoulders and upper limbs as the baby is forcibly extracted from the birth canal. This exerts traction on the upper cord of the brachial plexus, often causing a traction injury from which the baby can often recover. If the roots of C5 and C6 are avulsed from the spinal cord, the injury is permanent.