Following a penetrated injury in the submandibular triangle, the tongue of 45 year old patient detes to the left on protrusion. Which of the following nerves is injured?
**Core Concept**
The question is testing the student's knowledge of the motor innervation of the tongue, specifically the nerves responsible for its movement. The hypoglossal nerve (cranial nerve XII) is the primary motor nerve of the tongue, controlling its protrusion and movement.
**Why the Correct Answer is Right**
The correct answer is the hypoglossal nerve, which is responsible for the motor innervation of the extrinsic and intrinsic muscles of the tongue. The hypoglossal nerve is unique in that it is the only cranial nerve that has a purely motor function. It supplies the genioglossus, hyoglossus, and styloglossus muscles, which are responsible for tongue protrusion and movement. When the hypoglossal nerve is injured, the affected side of the tongue will deviate towards the injured side on protrusion.
**Why Each Wrong Option is Incorrect**
* **Option A:** The lingual nerve, a branch of the mandibular division of the trigeminal nerve (cranial nerve V), is responsible for sensory innervation of the tongue, including taste and sensation. It is not involved in the motor innervation of the tongue.
* **Option B:** The glossopharyngeal nerve (cranial nerve IX) is responsible for the sensory innervation of the posterior third of the tongue, including taste and sensation. It is not involved in the motor innervation of the tongue.
* **Option D:** The facial nerve (cranial nerve VII) is responsible for the motor innervation of the muscles of facial expression, including the muscles of the face and the stapedius and stylohyoid muscles. It is not involved in the motor innervation of the tongue.
**Clinical Pearl / High-Yield Fact**
In a patient with a tongue deviation, it is essential to examine the patient's ability to protrude the tongue to determine the affected side. A deviation of the tongue towards the injured side on protrusion is a classic sign of hypoglossal nerve injury.
**Correct Answer:** C. Hypoglossal nerve.