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The presence of an afferent pupillary defect, also known as a Marcus Gunn pupil, typically indicates a problem with the optic nerve or severe retinal damage in one eye. This defect occurs when light is shone in the affected eye, and the pupil responds inadequately compared to the unaffected eye. When light is shifted between the two eyes, the affected pupil may dilate instead of constricting, indicating a malfunction in the neural pathways that control pupil response.

Optic nerve disease, such as optic neuritis or glaucoma affecting the optic nerve, can lead to this type of defect due to a decrease in sensory input from the affected eye, causing an imbalance in light processing between the two eyes. Recognizing afferent pupillary defects can be critical in diagnosing underlying conditions affecting the optic nerve, which may require prompt intervention to prevent further vision loss.

In contrast, conditions like glaucoma, cataracts, and retinal tears do not typically result in an afferent pupillary defect but may cause other visual symptoms or changes. For instance, while glaucoma may lead to changes in peripheral vision and retinal tears could cause sudden vision loss, neither directly produces an afferent pupillary defect as indicative of optic nerve or severe retinal issues. Thus, the presence