Afferent Pupillary Defect (APD) or Marcus-Gunn pupil is a condition of the eye where the pupil doesn't dilate appropriately to the level of light reaching it. This will often result in one pupil appearing larger than the other (relative afferent pupillary defect - RAPD). This person with APD is often unaware of it except by looking in a mirror.
There are many causes of APD including Optic Neuritis (ON), glaucoma and optic nerve tumor. In multiple sclerosis, APD, is usually associated with damage to the optic nerve resulting from ON.
The reasons why afferent pupillary defect occurs with optic neuritis are complicated and involve the neuroanatomy of the visual pathways. Put simplistically, the eye in which the ON has occurred acts as if it is in a lower light situation than it really is and dilates more so that it can let more light hit the retina.
Afferent Pupillary Defect - Richmond Eye Associates