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: Gynaecology & Obstetrics
Combined oral contraceptives interfere with action of following drugs except 
Select and explain. 
A. Imipramine, B. Meperidine, C. Metoprotol, D. Metformin
D: Metformin
Answer: D: Metformin 
Explanation: Combined OCPs do not influence effectiveness of metformin.