A 43-year-old man is admitted to the hospital, having suffered a whiplash injury when his compact auto was struck from behind by a sports utility vehicle. MRI examination reveals some herniation of a disk in the cervical region. Physical examination reveals that the patient has lost elbow extension; there is absence of his triceps reflex and loss of extension of the metacarpophalangeal joints on the ipsilateral side. Which of the following spinal nerves is most likely affected?