Answer:
A Marcus Gunn pupil is also known as a relative afferent pupillary defect (RAPD) where there is decreased or no stimulation of the afferent visual system from the affected eye. This is usually indicative of a lesion which interrupts any part of the afferent visual pathway such as the retina, optic nerve, chiasm, or optic tract. Although cataracts do not typically result in this defect, a dense vitreous hemorrhage may cause RAPD.
A Marcus Gunn pupil can be tested for by shining a bright light starting with the unaffected eye which should result in the normal consensual constrictive response of both pupils and then moving the light over to the affected eye which will result in the case of RAPD in a paradoxical dilation of both eyes.