North American Veterinary Licensing Examination (NAVLE) Practice Test

What cranial nerve is damaged in a dog presenting with ptosis and facial drooping on one side?

1. CN VI

2. CN V

3. CN III

4. CN VII

The correct answer is: CN VII

The presence of ptosis and facial drooping on one side indicates an issue with the nerves responsible for eyelid elevation and facial muscle function. The cranial nerve that primarily controls the muscles of facial expression is the facial nerve, which is designated as cranial nerve VII. Damage to this nerve can lead to weakness or paralysis of the facial muscles on the affected side, resulting in facial drooping and inability to fully close the eye, which contributes to ptosis. Cranial nerve III (oculomotor nerve) is involved in eyelid elevation as well, but its damage would typically cause different ocular signs, such as pupil size abnormalities and positions of the eye, rather than predominantly affecting facial expression. Cranial nerve V (trigeminal nerve) is mainly responsible for sensory functions and mastication, and damage to this nerve would typically not cause the symptoms described. Cranial nerve VI (abducens nerve) controls lateral eye movement, and its impairment would primarily affect eye movement rather than facial expressions. In summary, the combination of ptosis and unilateral facial drooping strongly suggests dysfunction of cranial nerve VII, making it the correct answer.