A relative afferent pupillary defect (RAPD), also known as a Marcus Gunn pupil (after Robert Marcus Gunn), is a medical sign observed during the swinging-flashlight test[1] whereupon the patient's pupils excessively dilate when a bright light is swung from the unaffected eye to the affected eye.

The affected eye still senses the light and produces pupillary sphincter constriction to some degree, albeit reduced.

Depending on severity, different symptoms may appear during the swinging flash light test: Mild RAPD initially presents as a weak pupil constriction, after which dilation occurs.

[2] A second common cause of Marcus Gunn pupil is a contralateral optic tract lesion, due to the different contributions of the intact nasal and temporal hemifields.

[4][5] A total CN II lesion, in which the affected eye perceives no light, is very similar to a Marcus Gunn pupil; to distinguish them, in a CNII total lesion shining the light in the affected eye produces zero dilation nor constriction.