During cranial nerve examination, a neurologist asks her 33 year old patient to protrude his tongue. On doing so, her tongue detes to the right side. This finding results from paralysis of tongue muscle namely
**Core Concept**
During cranial nerve examination, the neurologist tests the function of the hypoglossal nerve (cranial nerve XII), which innervates the extrinsic and intrinsic muscles of the tongue. The hypoglossal nerve plays a crucial role in controlling the movement of the tongue, including protrusion, retraction, and elevation.
**Why the Correct Answer is Right**
The patient's tongue deviates to the right side upon protrusion, indicating weakness of the left side of the tongue. This is because the hypoglossal nerve on the left side is paralyzed, leading to the unopposed action of the right-sided tongue muscles. The right-sided muscles, which are still functioning, pull the tongue to the right side. The hypoglossal nerve controls the genioglossus, hyoglossus, and styloglossus muscles, which are responsible for protrusion, retraction, and elevation of the tongue, respectively.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the lingual nerve (a branch of the mandibular division of the trigeminal nerve) is responsible for sensation in the tongue, not motor function.
**Option B:** This option is incorrect because the facial nerve (cranial nerve VII) controls muscles of facial expression, including the buccinator and orbicularis oris, but not the tongue muscles.
**Option C:** This option is incorrect because the glossopharyngeal nerve (cranial nerve IX) is responsible for swallowing and sensation in the pharynx, but not motor function of the tongue.
**Clinical Pearl / High-Yield Fact**
The hypoglossal nerve is unique because it is the only cranial nerve that has a motor nucleus outside the brainstem, located in the medulla oblongata. This fact is useful to remember when examining patients with cranial nerve dysfunction.
**Correct Answer: C. Hypoglossal nerve.