Hi, jyyl. I think it's best to choose the subject you like. Medical schools usually give conditional offers that require 3 A's. Like myself, I did bio, chem, physics, and psychology last year. When I accepted my conditional offer, the condition is AAA for bio, chem and physics. I believe the medical school will choose your three best subjects to make a conditional offer, so it is quite unlikely that the choice of the fourth subject will affect your chance of getting admitted. After all, grades only represent the academic aspect. In fact, your work experience, personality will be taken into greater account.