A 32-year-old woman presents with a 2-week history of progressive blurred vision in the left eye and left-sided headache. On examination, the left pupil is 4 mm and reacts sluggishly to light, while the right pupil is 2 mm and reacts briskly. The left eye shows a relative afferent pupillary defect (RAPD). Visual acuity is 6/18 in the left eye and 6/6 in the right eye. Fundoscopy reveals left optic disc swelling. MRI brain shows a 2 cm lesion in the left optic nerve. What is the mechanism of the RAPD in this patient?