Which cranial nerve is tested when evaluating pupil constriction as part of eye function?

• A. Oculomotor nerve (CN III)
• B. Trigeminal nerve (CN V)
• C. Trochlear nerve (CN IV)
• D. Abducens nerve (CN VI)

Answer: (A)

Pupil constriction is controlled by parasympathetic signals carried by the oculomotor nerve. The nerve relays signals from the Edinger–Westphal nucleus to the ciliary ganglion, then via short ciliary nerves to the sphincter pupillae muscle, causing the pupil to constrict in response to light or near focus. Therefore, testing pupil constriction mainly assesses CN III function. If CN III is impaired, the pupil may remain dilated and show little or no constriction to light. The other nerves listed don’t control pupil size—they’re involved in eye movements (CN IV and CN VI) or facial sensation (CN V)—so they don’t directly mediate constriction.