Which cranial nerve would be most affected if eye movement commands were impaired and pupil constriction was affected?

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

Answer: (D)

Eye movement and pupil constriction rely on the same nerve: the one that supplies most of the extraocular muscles and carries parasympathetic fibers to the pupil. This nerve not only moves the eye but also controls the sphincter pupillae, which constricts the pupil, and the ciliary muscle, which adjusts lens shape for accommodation. When this nerve is impaired, you expect a combination of limited eye movements and a pupil that cannot constrict properly (often dilated and poorly reactive). The other nerves don’t provide both motor control of most eye movements and pupil constriction: one moves only the lateral rectus, another moves only the superior oblique, and the trigeminal is mainly sensory (and jaw/mastication muscles), not controlling pupil size. Thus, the nerve that best fits both deficits is the oculomotor nerve.