Your patient, Mr. Pratchett, is unable to open his eyelid.

Your patient, Mr. Pratchett, is unable to open his eyelid. On examination, you find that his pupil is dilated, and he is unable to move his affected eye in any direction except laterally and downward, indication that only his lateral rectus and superior oblique extrinsic eye muscles are functioning. Based on this information which cranial nerve do you think is involved in Mr. Pratchett's condition? Explain your reasoning.