Which cranial nerve is described as the oculomotor nerve and is involved in eye movement and pupil constriction?

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

Answer: (D)

The oculomotor nerve (cranial nerve III) is responsible for most eye movements and also carries parasympathetic fibers that constrict the pupil and accommodate the lens. It innervates the medial, superior, and inferior rectus muscles, the inferior oblique, and the levator palpebrae superioris, and it carries parasympathetic fibers from the Edinger-Westphal nucleus to the sphincter pupillae (pupil constriction) and ciliary muscle (lens accommodation). That combination—controlling several eye movements plus pupil constriction—fits the description perfectly. Other nerves listed either move the eye without pupil constriction (such as the abducens and trochlear) or are mainly sensory/motor to the face rather than eye movement (trigeminal). Clinical signs of CN III involvement include ptosis, a "down and out" eye, and pupillary dilation due to loss of parasympathetic input.