In Horner syndrome, the affected pupil is typically...

In Horner syndrome, the affected pupil is small and reacts briskly to light due to the disruption of sympathetic nerve pathways that normally promote pupil dilation. This condition is characterized by the classic triad of ptosis (drooping of the eyelid), miosis (constricted pupil), and anhidrosis (lack of sweating) on the side of the affected eye.

The small pupil occurs because the parasympathetic activity is unopposed, resulting in the constriction of the pupil. When exposed to light, this constricted pupil will still exhibit a brisk response—constricting even further in bright light, which is a normal reflex. Therefore, the response to light in the affected pupil remains intact, although the size is diminished compared to the unaffected pupil.

This understanding also helps clarify the other options. A pupil equal in size would not reflect the hallmarks of Horner syndrome, as the asymmetry in size is a key feature. A large and fixed pupil would suggest different pathophysiology, frequently associated with conditions like oculomotor nerve palsy. Similarly, a dilated pupil with a slow response does not align with the properties of the affected pupil in Horner syndrome, as it contradicts the characteristic miosis and