I think fifth pathway was originally meant for US citizens studying abroad in countries with a social service requirement after graduation from med school. In other words, lets say you went to med school in Mexico......instead of completing your social sevice year in that country, you could apply to a US med school and complete it there (theoretically). However, I believe only NYU really participates in this program. You would still be considered an IMG, but you would get the U.S. clinical experience needed to land a good residency, ofcourse, if you were a competitive candidate to begin with. Also, I think another pre-requisite is the completion of bachelors degree in the U.S.
In any case, you should check out usmle.org or ecfmg.org......I am sure they can help you further.