When medical students in the US take official licensing exams (or practice exams), they have to attest that they are not residents of Cuba, Crimea, Iran, North Korea, Syria, nor are they a national of Cuba or on a "list of designated parties maintained by the US government or otherwise blocked by the US government. Why is this list of countries significant? I recognize that some of them are communist nations that the US has conflict with but what about the others?
US Sanctions are run through the Treasury Dept, and the countries you listed are the ones that are under the most severe embargoes, which include blocking the school you're taking your test with from awarding a license to a resident of those countries or a national of Cuba.
The "list of designated parties" is probably the "Specially Designated Nationals and Blocked Persons List", which is a list of sanctions that don't apply to an entire country but to specific individuals.
There are other lists with different types of sanctions too, for example the "no fly list".
Of course we all know that these lists exist for all sorts of political reasons, so while there are international drug smugglers and shit on that list it's also a repository for everyone currently being punished for resisting American imperialism, such as the Cuban and North Korean people.