A 34-year-old man presents to your office with a 2-week history of headaches and double...

The inability to gaze laterally with the right eye indicates dysfunction of the right lateral rectus muscle , which is innervated by cranial nerve VI (the abducens nerve). The primary action of the lateral rectus muscle is abduction of the eye (movement away from the midline). Damage to the abducens nerve results in an inability to abduct the affected eye, leading to horizontal diplopia that worsens when looking toward the side of the lesion.

MCCQE objectives emphasize understanding cranial nerve function and localization. The oculomotor nerve (CN III) controls most extraocular movements but not lateral rectus. Trochlear nerve (CN IV) innervates the superior oblique muscle, affecting downward and inward gaze. The optic nerve (CN II) is responsible for vision, not eye movement. The trigeminal nerve (CN V) provides facial sensation and mastication motor function.

Headache with a new sixth nerve palsy may raise concern for increased intracranial pressure or intracranial pathology, as CN VI is vulnerable due to its long intracranial course. Further evaluation is warranted.