Final answer:
An afferent pupillary defect occurs when there is an asymmetry in the pupillary light reflex due to optic nerve damage or severe retinal disease, indicating an issue in the reflex's afferent pathway involving the optic nerve.
Step-by-step explanation:
When there is an afferent pupillary defect (APD), also known as a Marcus Gunn pupil, it indicates an asymmetry in the pupillary light reflex. During this reflex, the afferent pathway involves sensory input through the optic nerve, while the efferent pathway involves a motor response through the oculomotor nerve to the ciliary ganglion, which then affects the circular fibers of the iris. In normal conditions, both pupils constrict equally in response to light due to the direct and consensual reflexes. However, with an APD, when light is shone in the affected eye, the direct pupillary response is either absent or decreased compared to the consensual response seen when the other, unaffected eye is stimulated. The presence of an APD can suggest optic nerve damage or severe retinal disease in the affected eye.