A patient cannot move the right eye laterally and experiences diplopia when turning to the right. Which cranial nerve is affected?

• A. optic
• B. oculomotor
• C. trochlear
• D. abducens

Answer: (D)

The ability to move the eye outward (abduction) is carried by the lateral rectus muscle, which is innervated by the abducens nerve. If the right abducens nerve is damaged, the right lateral rectus can’t contract, so the right eye cannot abduct. When attempting to look to the right, the eyes should move together; with the right eye stuck medially due to unopposed medial rectus, the eyes become misaligned, producing diplopia. The optic nerve is a sensory pathway for vision and doesn’t control eye movements, the oculomotor nerve governs most other orbit muscles (and eyelid function), and the trochlear nerve controls the superior oblique, which is not responsible for primary lateral movement. In this scenario, the affected nerve is the abducens.