A 58-year-old man with a 10-year history of diabetes mellitus presents to the ophthalmology clinic with blurred vision and difficulty reading. On examination, the pupils are 4 mm in bright light and 6 mm in dim light. Direct light reflex is brisk and consensual reflex is intact. However, when a swinging flashlight test is performed, there is a subtle delay in the pupillary constriction to the light stimulus in the right eye compared to the left. Visual acuity is 6/9 in both eyes. Fundoscopy reveals scattered microaneurysms and hard exudates consistent with diabetic retinopathy. What is the most likely explanation for the pupillary finding?