This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Thorax, Subject name: Anatomy
[{'content': 'True about left phrenic nerve \nSelect and explain. \nA. Lies anterior to anterior scalenous muscle, B. Posterior to brachial plexus, C. Posterior to subclan aery, D. Posterior to hilum left lung', 'role': 'user'}, {'content': 'Answer: A \nExplanation: The phrenic nerves contain motor, sensory, and sympathetic nerve fibers. The phrenic nerve originates mainly from the 4th cervical nerve, but also receives contributions from the 5th and 3rd cervical nerves Both phrenic nerves run from C3, C4, and C5 along the anterior scalene muscle deep to the carotid sheath. The accessory phrenic nerve is a branch of the nerve to the subclavius Ref: BD Chaurasia; 7th edition', 'role': 'assistant'}]