This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Neck, Subject name: Surgery
[{'content': 'Which structure is preserved during modified radical neck dissection? \nSelect and explain. \nA. Phrenic nerve, B. Submandibular gland, C. Sternocleidomastoid, D. Thoracic duct', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Sternocleidomastoid muscle is preserved during modified radical neck dissection. Radical Neck Dissection Modified Radical Neck Dissection Removal of lymph nodes I-V + spinal accessory nerve + internal jugular vein + sternocleidomastoid muscleQ Removal of level I-V lymph nodes with (Mnemonic: SISm): Type I: preserves only Spinal accessory nerveQ Type II: preserves both spinal accessory nerve and Internal jugular veinQ Type III: preserves spinal accessory nerve, internal jugular vein and Sternocleidomastoid muscleQ(Functional neck dissectionQ)', 'role': 'assistant'}]