This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Anatomy
[{'content': 'Not a middle mediastinal structure ? \nSelect and explain. \nA. Pulmonary trunk, B. Phrenic nerve, C. Thymus, D. Hea', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Thymus', 'role': 'assistant'}]