In a patient it is noticed that when he protrudes his tongue it deviates to the left. The nerve damaged is:
**Core Concept**
The patient's symptoms suggest a unilateral weakness of the tongue muscles, which is a classic indicator of a cranial nerve lesion. The cranial nerves involved in tongue movement are the hypoglossal nerve (CN XII) and the lingual nerve (a branch of CN XII). However, the primary motor control of the tongue is mediated by the hypoglossal nerve.
**Why the Correct Answer is Right**
When the hypoglossal nerve is damaged, the affected side of the tongue will deviate towards the side of the lesion when protruded. This occurs because the intact side of the tongue will push the weakened side towards the midline. The hypoglossal nerve supplies all the extrinsic and intrinsic muscles of the tongue, except for the palatoglossus muscle, which is innervated by the vagus nerve (CN X). The hypoglossal nerve's motor fibers originate from the nucleus of the hypoglossal nerve in the medulla oblongata.
**Why Each Wrong Option is Incorrect**
* **Option A:** The vagus nerve (CN X) is responsible for the motor innervation of the pharynx and larynx, but not the tongue. Damage to the vagus nerve would not cause tongue deviation.
* **Option B:** The facial nerve (CN VII) is involved in the motor innervation of the muscles of facial expression, but not the tongue. Damage to the facial nerve would not cause tongue deviation.
* **Option C:** The glossopharyngeal nerve (CN IX) provides sensory innervation to the posterior third of the tongue, but not motor innervation. Damage to the glossopharyngeal nerve would not cause tongue deviation.
**Clinical Pearl / High-Yield Fact**
Tongue deviation is a classic sign of hypoglossal nerve damage, and it can be remembered using the phrase "weak side out" to indicate that the weakened side of the tongue will deviate towards the side of the lesion.
**Correct Answer:** C.