skip to main content

Parasympathetic Eye Miosis — FRCA Primary MCQ

Instant feedback + full explanation. One question, done properly.

HardPhysiology - NeurophysiologyParasympathetic Eye MiosisFRCA Primary

A third-nerve compression affects parasympathetic fibres before somatic motor fibres. Which pupil finding is most likely on the affected side?

Educational content. Not a substitute for clinical judgement or local policy.

Reveal the answer and explanation

Correct answer: A — A dilated pupil with reduced direct and consensual light constriction

The best answer is “A dilated pupil with reduced direct and consensual light constriction”. Parasympathetic fibres travelling with the third nerve supply the sphincter pupillae after synapsing in the ciliary ganglion. Their interruption leaves sympathetic dilator activity unopposed, causing mydriasis and impaired constriction when that eye is illuminated or when the opposite eye is illuminated. An afferent optic-nerve lesion, not a third-nerve lesion, gives a different light-response pattern.

Reference: RCoA, Primary FRCA syllabus: https://www.rcoa.ac.uk/sites/default/files/documents/2026-01/Primary-FRCA-Syllabus.pdf; Microvascular Non-Arteritic Ocular Motor Nerve Palsies: https://pmc.ncbi.nlm.nih.gov/articles/PMC5123092/