The Nasdaq led the stock market lower Monday as medical stocks were routed. The composite closed with a loss of 0.6%, hurt in large part by biotechs. That top-performing industry group slid broadly. The iShares Nasdaq Biotechnology (IBB) exchange traded fund plunged more than 4%. Generic-drug company Mylan (MYL) rejected Teva’s (TEVA) buyout bid, which seemed to have a deflating effect on the health-care sector.