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

• A. Horner's syndrome
• B. Adie's tonic pupil
• C. CN 3 palsy
• D. Pharmacological dilation

Answer: (A)

Pupil size is determined by a balance between sympathetic dilation and parasympathetic constriction. A mydriatic pupil is dilated and often poorly reactive to light, reflecting disruption of constriction or excess dilation.

Horner's syndrome arises from loss of sympathetic input to the eye, which actually produces a constricted pupil (miosis) with ptosis. Anisocoria is typically greater in dim light because the affected pupil cannot dilate normally, but the pupil itself is not dilated. So this condition does not cause a mydriatic pupil.

Adie's tonic pupil involves postganglionic parasympathetic dysfunction leading to a dilated pupil that constricts slowly and incompletely, often with accommodation relatively preserved. This is a mydriatic finding.

CN III palsy damages the oculomotor nerve, including the parasympathetic fibers to the sphincter pupillae, resulting in a dilated, poorly reactive pupil along with ptosis and eye misalignment.

Pharmacological dilation directly expands the pupil with topical agents, producing a clearly mydriatic pupil.

So the finding that does not fit a mydriatic pupil is Horner's syndrome, since it characteristically causes miosis rather than dilation.