A 27-year-old man was struck on the head at football a week ago. He now sees two images, one above the other, worst when reading or going down stairs. The vertical separation is greatest on looking down and to the left, and increases when he tilts his head to the right. Covering the right eye abolishes the lower image. Both eyes abduct fully, the eyelids are level and the pupils react equally.
Damage to which one of the following cranial nerves accounts for the pattern of this man's double vision?