What is afferent pupillary defect?

Marcus-Gunn pupil: An afferent pupillary defect indicates a lesion in the afferent limb (retina or optic nerve) of the pupillary reflex. Such a lesion is best appreciated on the "swinging flashlight test." a lesion in the left optic nerve will decrease the perceived illumination in that eye, and when the flashlight is swung from the right to the left eye, the left pupil will appear to dilate slightly.