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: Pharmacology
[{'content': "Which of the following statements best describes an 'orphan drug' ? \nSelect and explain. \nA. It is a drug which acts on orphanin receptors, B. It is a very cheap drug, C. It is a drug which has no therapeutic use, D. It is a drug required for treatment or prevention of a rare disease", 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]