The Marcus Gunn pupil is a condition characterized by an abnormal response of the pupil to light. When light is shone in the affected eye, the pupil may constrict less than expected or even dilate, while the other pupil responds normally. This phenomenon occurs due to a defect in the sensory pathway, often associated with conditions like optic nerve damage or retinal detachment. This condition is named after Sir Marcus Gunn, a British ophthalmologist who first described it. The Marcus Gunn pupil is typically assessed during a neurological examination, where doctors use a light reflex test to observe the pupils' reactions. Proper diagnosis is essential for determining the underlying cause and appropriate treatment.