Afferent pupillary defect is seen in: March 2012

Ans: D i.e. Retrobulbar neuritis When the input, through both the optic nerves to the midbrain is equal, the pupils to which the light is transferred will remain tightly constricted since the consensual response has the same magnitude as a direct response. Should there be a lesion of one optic nerve, the input from that side is less than that from the normal side. In that case when the light is transferred to the diseased eye both pupils will dilate, & on swinging back to the normal side, both pupils will constrict. The dilatation of 'escape' that occurs is commonly called the Marcus Gunn pupil or an afferent pupillary defect & may be the earliest indication of an optic nerve disease such as retrobulbar neuritis