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 is an active metabolite of another drug and is available as a separate drug for the treatment of angina ? 
Select and explain. 
A. Isosorbide mononitrate, B. Isosorbide dinitrate, C. Nitroglycerine, D. Propranolol
A: Isosorbide mononitrate
Answer: A: Isosorbide mononitrate 
Explanation: None