I am in this identical situation (except I studied math and CS). I didn't exactly give them a reason for why I went to work... I suppose the reason would be "because that's what you do after you graduate." It wasn't until I realized that I hated software development, and that medicine has a very large number of things that are the exact opposite of the aspects of software development I hated, that I realized I needed to pursue an MD. (Plus, as cliche as it is to assert it, I really did always consider going into medicine.)
If you're going to explain "why" anything, it should be what caused you to change your mind and go into medicine, not what caused you to go to work in the first place.