I disagree with Dr. K, as I know numerous people who graduated from Caribbean schools and went through programs that are highly competitive.
But he is right about the ECFMG. The USMLE is an exam that has to be taken by anyone intending to practice in the US. It doesn't matter where you are educated--even US students have to take it.
Due to the variability in the quality of education in Caribbean schools, you want to be sure that the state you intend to practice in recognizes the school. Many states simply state that they review each application for licensure on a case by case basis, but that's not really true. There are some schools that are unacceptable. The most comprehensive list of unacceptable schools is maintained by Oregon's Office of Degree Authorization:
http://www.osac.state.or.us/oda/unaccredited.aspxMany states follow the California approved school list. The last time I checked, there are only 4 Caribbean schools approved by the state of California and in essence, if California approves the school, all of the states will accept the school. This is their approved schools list:
http://www.medbd.ca.gov/applicant/schools_recognized.htmlAnd this is their unapproved schools list:
http://www.medbd.ca.gov/applicant/schools_unapproved.htmlThe lax standards of the Caribbean schools are well known by state medical boards. Some states are requiring applicants to show their passports to prove they were on the island when classes were in session. (I know of several people who only came to the schools to take tests and returned home to study--not acceptable to the state boards).
Some schools don't require all of the prerequisites that US schools do. There are several states that won't license applicants if they haven't completed all of the undergraduate pre-reqs. Your best move is to review your state medical board's licensing requirements. All states post them. Here's a link that will provide a link to all 50 states' medical boards:
http://www.fsmb.org/directory_smb.html