Question: What’s a Marcus-Gunn pupil?

Answer: This is simply a relative afferent pupillary defect (RAPD). When one eye doesn’t see light as well, then the eyes constrict less when light is shined in that eye.

This can be a subtle lack of contriction, so it’s helpful to compare the constriction using the other “good” eye as a control. We do this with the swinging light test.