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
Which of the following statements best describes an 'orphan drug' ? 
Select and explain. 
A. 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
D: It is a drug required for treatment or prevention of a rare disease
Answer: D: It is a drug required for treatment or prevention of a rare disease 
Explanation: None