A 23-year-old woman is noticed to have irregular pupils on routine examination. She has one large pupil, which has minimal response to light stimulation, but it does respond to accommodation. The eye movements are normal and she experiences no double vision.For the above patient with a pupillary abnormality, select the most likely diagnosis

The tonic pupil (Holmes-Adie syndrome) is caused by a parasympathetic lesion at or distal to the ciliary ganglion. The pupil is large and usually unilateral, with absent response to light. A bright room, by causing constriction of the normal pupil, accentuates the anisocoria. The tonic pupil can be associated with Shy-Drager syndrome, amyloidosis, or diabetes. However, it is most commonly seen in otherwise healthy young women.