What is an important clinical test for measuring facial nerve function?

Measuring facial nerve function often involves assessing the ability to perform movements of the facial muscles, which are directly innervated by the facial nerve (cranial nerve VII). One critical clinical test for evaluating this function is the ability to close the eyes tightly. This action engages several muscles of facial expression, specifically the orbicularis oculi muscle, which is responsible for closing the eyelids. If a patient is unable to close their eyes tightly, it may indicate weakness or paralysis of the facial nerve on the affected side, commonly seen in conditions such as Bell's palsy or after a stroke.

The other choices are associated with different cranial nerves or functions; for instance, turning the head against resistance primarily evaluates the accessory nerve (cranial nerve XI), while Rinne's test assesses auditory function associated with the vestibulocochlear nerve (cranial nerve VIII). Phonating "ah" evaluates the functioning of the vagus nerve (cranial nerve X) and the hypoglossal nerve (cranial nerve XII) for speech capabilities, rather than the facial nerve. Thus, the ability to close the eyes tightly is a specific and relevant measure for assessing facial nerve function.