## Core Concept
The question tests the understanding of the innervation of the tongue, specifically the motor innervation that controls tongue movements. The deviation of the tongue upon protrusion is a clinical indicator of weakness in the tongue muscles on one side.

## Why the Correct Answer is Right
When the tongue is protruded and deviates to one side, it indicates weakness of the muscles on the side to which the tongue deviates. This is because the weakened muscles are unable to counterbalance the push from the healthy side during protrusion. The **hypoglossal nerve (CN XII)** provides motor innervation to all the intrinsic and extrinsic muscles of the tongue except for palatoglossus. Damage to one hypoglossal nerve results in weakness of the ipsilateral tongue muscles, leading to deviation of the tongue towards the side of the lesion upon protrusion. Therefore, if the tongue deviates to the right, the **right hypoglossal nerve** is damaged.

## Why Each Wrong Option is Incorrect
- **Option A:** The **mandibular division of the trigeminal nerve (V3)** primarily provides sensory innervation to the lower face and motor innervation to the muscles of mastication. It does not control tongue movements.
- **Option B:** The **glossopharyngeal nerve (CN IX)** provides sensory innervation to the posterior one-third of the tongue and motor innervation to stylopharyngeus. It does not control tongue protrusion or deviation.
- **Option C:** The **vagus nerve (CN X)** provides motor innervation to the laryngeal and pharyngeal muscles (except for stylopharyngeus) and sensory innervation to the larynx and pharynx. It does not directly control tongue movements.
- **Option D:** This option is not provided, but based on the correct answer being , we can infer that the focus is on the hypoglossal nerve.

## Clinical Pearl / High-Yield Fact
A useful clinical correlation to remember is that when assessing a patient with suspected unilateral tongue weakness, asking the patient to protrude their tongue and observing for deviation can help localize a lesion. A lesion affecting the **hypoglossal nerve (CN XII)** will cause the tongue to deviate towards the side of the lesion.

## Correct Answer: . hypoglossal nerve