Mayo Foundation for Medical Education & Research v. United States, 131 S.Ct 704 (U.S. 2011)

(Chevron U.S.A. Inc. v. Natural Resources Defense Council, Inc., 467 U.S. 837 (1984) is the appropriate review standard under which Treasury Regulations underlying an ambiguous Code provision are to be evaluated; using the Chevron standard, Treas. Reg. Sec. 31.3121(b)(10)-2 upheld with result that medical residents are to be considered employees rather than students and, as such, cannot receive exemption from Social Security tax).