This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Salivary Glands, Subject name: Surgery
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: 
Select and explain. 
A. Parotid duct, B. Facial nerve temporal branch, C. Facial nerve cervical branch, D. Facial nerve main trunk
C: Facial nerve cervical branch
Answer: C: Facial nerve cervical branch 
Explanation: The cervical branch of the facial nerve innervates the lower lip through the marginal mandibular branch of the nerve . As no cross innervation exits to other branches of the facial nerve, marginal mandibular branch injuries always yield paralysis of the same side of the lower lip. Injuries of the main trunk of the facial nerve or its temporal branch would usually produce upper facial paralysis as well.