The U.S. Supreme Court on Tuesday upheld an Internal Revenue Service requirement that medical residents pay Social Security taxes. Full-time students who work are generally exempt from Social Security taxes, but law clerks and tradesman apprentices are not. The high court, in an unanimous opinion by Chief Justice John Roberts, upheld a Treasury Department rule that said medical residents are full-time employees, not students. Justice Roberts said the department's interpretation was a reasonable one, rejecting arguments by the Mayo Clinic that the rule was arbitrary. "The department certainly did not act irrationally in concluding that these doctors. . . are the kind of workers that Congress intended to both contribute to and benefit from the Social Security system," Justice Roberts wrote in a 15-page opinion.