When a light is shone into the pupil, which response is expected?

• A. the opposite pupil should dilate
• B. the pupil should become larger in size
• C. both pupils should dilate together
• D. the pupil should become smaller

Answer: (D)

In the pupillary light reflex, bright light prompts the pupil to constrict. Shining light into one pupil triggers a direct constriction in that eye and a consensual constriction in the opposite pupil as well. So the expected change is the pupil becoming smaller. This happens because the light signal travels from the retina to the midbrain, activating the Edinger-Westphal nuclei, which send parasympathetic signals to the iris sphincter muscle to constrict. The other options describe dilation or dilation in only one eye, which doesn’t fit this reflex.