A 23-year-old woman is noticed to have irregular pupils on routine examination. She has 1 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.