Miosis is best described as which pupillary change?

• A. Pupillary dilation due to sympathetic activity.
• B. Equal pupil size in bright light due to optic nerve.
• C. Pupillary constriction due to parasympathetic CN III releasing acetylcholine.
• D. Pupil dilation due to external light.

Answer: (C)

Miosis is the constriction of the pupil produced by parasympathetic activation of the sphincter pupillae muscle via the oculomotor nerve. Parasympathetic fibers originate in the Edinger-Westphal nucleus, travel with the oculomotor nerve to the ciliary ganglion, then through short ciliary nerves to release acetylcholine onto muscarinic receptors on the sphincter pupillae. This causes the muscle to contract and the pupil to become smaller, a response especially important in bright light to protect the retina and help focus. The optic nerve is involved in sensing light but does not directly drive the constriction, which is why a description attributing constriction to parasympathetic CN III releasing acetylcholine is the best fit. Dilation describes the opposite process controlled by sympathetic pathways, and a statement about equal pupil size in bright light doesn’t capture the motor mechanism of miosis.