If you take the MCAT in August, schools will be able to consider your primary in September. That is too late for MD-PhD, even assuming you get a 520+ score. I submitted 5 secondaries in September (I had already submitted to 12 schools prior to that) and got immediately rejected at 4 of them, even though they were all much less competitive than the first 12 I applied to. In comparison I got interviews from 10 out of the first 12. You should apply next cycle.