Marcus Gunn pupil is due to -

The Relative Afferent Pupillary Defect (RAPD), or Marcus-Gunn Pupil is an extremely significant and highly objective clinical finding in the examination of the visual system. The "swinging flashlight test" is probably the best test for identifying an RAPD. In this test, a strong, steady light is used. The light is shined into one eye, and then quickly switched to the other. This is repeated back and foh, until one of four conclusions is reached (listed below). Since light in one pupil causes both pupils to constrict, quickly switching from one eye to the other will give a "relative" indication of the functioning of each eye and optic nerve. If both eyes are equally dysfunctional, no "relative" defect would be found.