FM is 3 years in both the DO and MD world.
the second post is correct, if you do a DO internship and then apply for the MD resid, you will have to get that intern year approved toward the total 3 yrs. but if you stay DO, one year intern, two yrs FM. so, they are both three years. basically it is one of the fields where the rotating intern is so much like FM, of course it would count toward the resident years.
the fields we do more "time" , for example is OB/GYN. it is a 4 yr program DO/MD. but if you go DO, you have one year prior that is intern, total five yrs ( oh, there are some fast track programs. . .a lot of stress but cuts off one yr)
hope this helped
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ob/gyn wannabe