Which of the following is NOT an etiology of a miotic pupil?

• A. Horner's syndrome
• B. Argyll Robertson pupil
• C. CN 3 palsy
• D. Uveitis with posterior synechiae

Answer: (C)

Pupil size is determined by the balance between parasympathetic constriction and sympathetic dilation. A miotic pupil results from conditions that enhance constriction or reduce dilation.

Horner's syndrome causes miosis because loss of sympathetic input allows unopposed parasympathetic activity to constrict the pupil. Argyll Robertson pupil is small and constricts with accommodation but not with light, so it remains miotic in many contexts. Uveitis with posterior synechiae produces a small pupil due to inflammation and adhesions that pull the iris and sphincter constrictor together, narrowing the pupil.

In contrast, CN III palsy disrupts the parasympathetic fibers that drive constriction, leading to a dilated pupil rather than a miotic one. Therefore, CN III palsy is not an etiology of a miotic pupil.