(a)General certification requirements. (1) Except as provided in paragraph (b) or paragraph (d)(1) of this section, any alien who seeks admission to the United States as an immigrant or as a nonimmigrant for the primary purpose of performing labor in a health care occupation listed in paragraph (c) of this section is inadmissible unless the alien presents a certificate from a credentialing organization, listed in paragraph (e) of this section. (2) In the alternative, an eligible alien who seeks to