This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: C.N.S., Subject name: Medicine
A 60-year-old man is brought to the emergency department because of new symptoms of double vision and discomfort in his left eye. He reports no other neurologic symptoms. On physical examination, there is ptosis of the left eyelid, the eye is rotated down and out, and the pupil is 3 mm and reactive to light. The right eye is normal, and there are no other focal neurological deficits. Which of the following is the most likely diagnosis? 
Select and explain. 
A. fourth nerve palsy, B. diabetic autonomic neuropathy, C. third nerve palsy, D. sixth nerve palsy
C: third nerve palsy
Answer: C: third nerve palsy 
Explanation: Third nerve palsy can result in ptosis of the eyelid. There is also loss of the ability to open the eye, and the eyeball is deviated outward and slightly downward. With complete lesions, the pupil is dilated, does not react to light, and loses the power of accommodation. In diabetes, the pupil is often spared. The sixth cranial nerve can also be affected by diabetes, but this is much less common.