If you shine a light into one eye, what is the expected direct reflex in the same eye?

If you shine a light into one eye, what is the expected direct reflex in the same eye?

• A. Dilate
• B. Remain unchanged
• C. Constrict
• D. Move

Answer: (C)

Shining light into one eye triggers the direct pupillary light reflex, so that eye’s pupil constricts. This happens because the retina detects the light and sends signals to the midbrain, where the Edinger–Westphal nucleus activates parasympathetic fibers through the oculomotor nerve to the sphincter pupillae muscle, causing constriction. The other eye typically constricts as well (consensual reflex), but the question focuses on the illuminated eye. Dilation would be the opposite response, remaining unchanged isn’t a reflex, and moving the eye is not related to pupil size.

Shining light into one eye triggers the direct pupillary light reflex, so that eye’s pupil constricts. This happens because the retina detects the light and sends signals to the midbrain, where the Edinger–Westphal nucleus activates parasympathetic fibers through the oculomotor nerve to the sphincter pupillae muscle, causing constriction. The other eye typically constricts as well (consensual reflex), but the question focuses on the illuminated eye. Dilation would be the opposite response, remaining unchanged isn’t a reflex, and moving the eye is not related to pupil size.