Final answer:
The cranial nerve being tested when a patient is asked to read the Snellen chart is the Optic nerve (CN II).
Step-by-step explanation:
The cranial nerve that is being tested when a patient is asked to read the Snellen chart is the Optic nerve (CN II).
The Snellen chart is used to measure visual acuity, which is the ability to see clearly. The optic nerve is responsible for transmitting visual information from the eye to the brain. By reading the letters on the Snellen chart, the patient's ability to perceive and distinguish different sizes of letters at a specific distance can be assessed, helping to diagnose any issues with visual acuity.
Other cranial nerves involved in vision include the Oculomotor nerve (CN III) for eye movement, the Trochlear nerve (CN IV) for downward and inward eye movement, and the Abducens nerve (CN VI) for outward eye movement.