Which nerve is tested by asking the patient to smile and frown?

The facial nerve is responsible for controlling the muscles of facial expression, which includes the ability to smile and frown. When a patient is asked to perform these actions, it allows healthcare professionals to assess the functioning of the facial nerve, as it innervates the muscles that create these expressions. The direct involvement of the facial nerve in controlling facial movements makes it the correct choice in this context.

The trigeminal nerve primarily handles sensations in the face and is responsible for the motor functions of mastication (chewing), rather than facial expressions. The glossopharyngeal nerve mainly serves functions related to taste and swallowing, while the vagus nerve has a broad range of functions related to the autonomic nervous system and does not directly control facial expressions. Therefore, the facial nerve is uniquely positioned in this scenario to be tested through the actions of smiling and frowning.