###Rationale: Horner syndrome results in a small, round pupil on one side. Light and near reaction is brisk, and response to mydriatics and miotics is normal. The affected pupil will not dilate in the dark, so darkness accentuates the anisocoria. The syndrome is often idiopathic but can be caused by neoplasm, brain stem stroke, or carotid dissection.

###Answer: OPTION B IS CORRECT.
###Question: A 63-year-old man is noticed to have asymmetric pupils. He is completely well and has no symptoms. On examination, the left pupil is small, round, and has a brisk response to light and near stimuli (accommodation). There is also ptosis of the left eyelid, but no other ocular movement abnormality or symptoms of double vision.For the above patient with a pupillary abnormality, select the most likely diagnosis.
###Options:
A. essential anisocoria
B. Horner syndrome
C. tonic pupils (Holmes-Adie syndrome)
D. Argyll Robertson pupils
