Which nerve is primarily tested by visual acuity charts (Snellen chart)?

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

Answer: (D)

Visual acuity testing with a Snellen chart checks the afferent pathway of vision—the retina’s ability to receive light and the optic nerve’s ability to carry that sensory information to the brain. The chart measures how clearly a person can resolve details, which hinges on the optic nerve transmitting those visual signals to the visual cortex. If the optic nerve is impaired, the signals don’t reach the brain properly, and acuity is reduced, even if the eye can focus light normally. The other nerves listed mainly control eye movements or facial sensation and do not form the primary sensory pathway for reading the chart, although poor alignment from motor nerve issues can affect results indirectly.