Mrs. Lopez complains of a persistent weakness in flexion at the elbow (flexion of the forearm) and has trouble supinating her forearm. In addition to feeling as though her strength is not sufficient, she has trouble positioning her arm to carry her purse with the affected limb. That said, when asked to flex her wrist and digits, there is no difference in strength between the affected and unaffected limb. What nerve has been injured in Mrs. Lopez?