If any graduate of a foreign medical school expects to be able obtain residencies and become licensed to practice in the United States, they must have earned the medical degree at a medical school with a listing in the World Directory of Medical Schools, a UN World Health Organization (WHO) publication.
Each government in the world is asked to submit to WHO a list of their recognized medical schools. For the US and Puerto Rico that list consists only of medical schools accredited by the Association of American Medical Colleges(AAMC). There are four medical schools in Puerto Rico, but only three are AAMC accredited. Graduates of the fourth school, recognized only by the Commonwealth of PR, can be licensed to practice in PR, but not in the 50 states.