Relative Afferent Pupillary defect (RAPD) occur due to damage to which of the following structures?

Marcus Gunn Pupil indicates relative afferent pupillary defect (RAPD) and it is caused by incomplete optic nerve lesions or severe retinal diseases. Marcus Gunn Pupil may be present as early as someone with complete vision in optic nerve disease. Ref: Ophthalmology By A.K Khurana, 4th Edition, Page 292; Textbook of Ophthalmology By Sunita Agarwal, Athiya Agarwal, David J Apple, 2002, Page 303, 304