A 43-year-old teacher underwent left parotidectomy. Upon awakening from surgery, paralysis of the left lower lip was observed. This complication was most likely due to injury to which of the following:
**Core Concept**
The facial nerve is a complex nerve that controls the muscles of facial expression, taste, and other functions. The nerve is divided into five sections, with the mandibular division (or inferior division) being responsible for the motor innervation of the muscles of the lower lip.
**Why the Correct Answer is Right**
The nerves responsible for the motor innervation of the muscles of the lower lip are branches of the mandibular division of the facial nerve. The nerve to the medial pterygoid muscle and the nerve to the lateral pterygoid muscle are branches of the mandibular division, but the motor innervation of the muscles of the lower lip is primarily provided by the nerve to the buccinator muscle and the nerve to the inferior auricular muscle. However, the motor innervation of the muscles of the lower lip is primarily provided by the nerve to the depressor anguli oris and the nerve to the depressor labii inferioris, which are branches of the mandibular division of the facial nerve. Injury to these branches during a parotidectomy can lead to paralysis of the lower lip.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not provided.
* **Option B:** The buccal branch of the facial nerve is responsible for the motor innervation of the buccinator muscle and the muscles of facial expression in the cheek, not the muscles of the lower lip.
* **Option C:** The marginal mandibular branch of the facial nerve is a branch of the mandibular division of the facial nerve, but it primarily innervates the muscles of the lower lip, including the depressor anguli oris and the depressor labii inferioris. Injury to this branch can lead to paralysis of the lower lip.
**Clinical Pearl / High-Yield Fact**
The marginal mandibular branch of the facial nerve is at risk of injury during a parotidectomy, resulting in paralysis of the lower lip. This is a common complication of this procedure.
**Correct Answer:** C.