What action is performed to assess cranial nerve X (vagus nerve)?

Cranial nerve X, known as the vagus nerve, plays a significant role in various functions including speech and swallowing. To assess the vagus nerve, the action of phonating "ah" is particularly relevant since it allows for the evaluation of the movement of the soft palate and the quality of the voice.

When a person phonates "ah," the clinician can observe the symmetry of the uvula. A deviation of the uvula to one side may indicate dysfunction in one of the branches of the vagus nerve which innervates the muscles responsible for elevating the soft palate. Additionally, the voice quality can help assess for issues such as hoarseness, which may further suggest a problem involving the vagus nerve or its branches.

Other options do not directly assess the vagus nerve: protruding the tongue primarily evaluates cranial nerve XII (hypoglossal nerve), clenching teeth focuses on cranial nerve V (trigeminal nerve), and assessing hearing acuity pertains to cranial nerve VIII (vestibulocochlear nerve). Thus, phonating "ah" successfully targets the vagus nerve and provides valuable information about its function.