Which cranial nerve carries afferent input for the pupillary light reflex?

• A. Oculomotor nerve (CN III)
• B. Trochlear nerve (CN IV)
• C. Trigeminal nerve (CN V)
• D. The optic nerve (CN II)

Answer: (D)

The afferent input for the pupillary light reflex comes from the retina and travels through the optic nerve to the midbrain. From there, signals reach the pretectal area and bilaterally activate the Edinger–Westphal nucleus, whose parasympathetic output travels with the oculomotor nerve to the sphincter pupillae to induce constriction. So the optic nerve is responsible for sensing the light and initiating the reflex.

The oculomotor nerve is involved in carrying the efferent, constrictor signal that tightens the pupil; the trochlear nerve controls a motor eye muscle (not this reflex); and the trigeminal nerve provides sensory input for other facial reflexes like the corneal reflex, not the direct afferent limb of the pupillary light reflex.