In the swinging flashlight test, both pupils constrict when light shines in the normal left eye, but the right pupil paradoxically dilates (re-dilates) when light is swung to it. This indicates:

• A A right efferent pupillary defect (third nerve palsy)
• B Horner's syndrome on the right
• C A left afferent pupillary defect
• D A relative afferent pupillary defect in the right eye due to optic nerve disease ✓

Explanation

The relative afferent pupillary defect (RAPD or Marcus Gunn pupil) is detected by the swinging flashlight test. When light moves from a healthy eye to a diseased eye, the diseased eye receives a weaker afferent signal, so both pupils dilate (the consensual constriction from the healthy eye is withdrawn). RAPD indicates an asymmetry in the afferent visual pathway—typically optic nerve disease, optic tract pathology, or severe retinal disease—and is one of the most clinically important signs in neuro-ophthalmology.

Reference: Khurana Comprehensive Ophthalmology, 7th ed.

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Written and medically reviewed by the StethoPrep medical team.