Which nerve mediates the efferent limb of the pupillary light reflex to constrict the sphincter pupillae?

• A. Trochlear nerve (CN IV) to ciliary muscle.
• B. Optic nerve (CN II) to iris.
• C. Abducens nerve (CN VI) to sphincter pupillae.
• D. Oculomotor nerve (CN III) to ciliary ganglion to sphincter pupillae.

Answer: (D)

The efferent limb of the pupillary light reflex is carried by the oculomotor nerve. Parasympathetic fibers originate in the Edinger–Westphal nucleus, travel with the oculomotor nerve to the ciliary ganglion, and then postganglionic fibers ride with the short ciliary nerves to the sphincter pupillae to induce constriction in response to light. In the reflex arc, the optic nerve provides the afferent signal from the retina, not the efferent constriction pathway. The trochlear and abducens nerves control extraocular muscles and do not mediate pupil constriction.