This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Other topics and Adverse effects, Subject name: Anatomy
Which of the following is an active metabolite of another drug and is available as a separate drug for the treatment of angine? 
Select and explain. 
A. Isosorbide mononitrate, B. Isosorbide dinitrate, C. Nitroglycerine, D. Propanolol
A: Isosorbide mononitrate
Answer: A: Isosorbide mononitrate 
Explanation: (Ref: KDT 6/e p525) It is a metabolite of isosorbide dinitrate and is also available as a separate drug. It undergoes least first pass metabolism.