A 13-year-old boy is brought to the emergency depament after losing control during a motorbike race in which he was run over by several of the other racers. Physical examination reveals several cuts and bruises.He is unable to extend the left wrist, fingers, and thumb, although he can extend the elbow. Sensation is lost in the lateral half of the dorsum of the left hand. Which of the following nerves has most likely been injured to result in these signs, and in what pa of the arm is the injury located?

The radial nerve innervates the extensor compaments of the arm and the forearm. Supplies the triceps brachii proximal to the spiral groove so elbow extension is intact. Provides sensory innervation to much of the posterior arm and forearm as well as the dorsal thumb, index, and middle fingers up to the level of the fingernails. Symptoms are described only in the distal limb due to the midhumeral location of the lesion. Median nerve innervates flexors of the forearm and thenar muscles and provides sensory innervation to the lateral palmar hand. Ulnar nerve supplies only the flexor carpi ulnaris and the medial half of the flexor digitorum profundus in the forearm.