MCAT score is the key. I know one fresh PhD graduate of biology (american kid), he wanted to go to MD school but did not have good MCAT score, he applied but got 0 interview. In addition, as drotom mentioned, MD school candidates usually have to have volunteering experiences in a clinical setting, to show committment to patient care. Lab experience is also a plus, but with less weight to application package relative to that of clinical one.