A 24-year-old student is brought to the emergency room after being found in a ditch where he had lain overnight after being hit by a car. He complains of severe pain in the left arm and
examination reveals a broken humerus. Neurological examination reveals that the patient can extend the elbow but displays inability to supinate the elbow when it is extended. The patient also has wrist drop and very weak hand grasp. The neurological lesion is likely localized at which of the following locations?
(A) posterior cord of the brachial plexus
(B) posterior divisions of the brachial plexus
© radial nerve at the distal third of the humerus
(D) radial nerve at the midforearm
(E) radial nerve at the wrist
explanation: © Wrist drop and weak hand grasp indicate that the patient has a lesion of the radial nerve, most likely at the distal third of the humerus, sparing the innervation of the tríceps brachii but affecting the supinator. Because the patient can extend the elbow, the integrity of the posterior cord (choice A) and posterior divisions (choice B) of the brachial plexus is preserved. Lesion of the radial nerve at the midforearm
(choice D) and wrist (choice E) would spare the innervation of the supinator.