 Sarepta Therapeutics Inc. SRPT, 5.14 shares plummeted as much as 20 in after-hours trading Thursday, after the company reported that a study had been put on hold by the Food and Drug Administration. Sarepta disclosed that a patient in a study of a proposed drug for Duchenne muscular dystrophy suffered a serious adverse event of hypomagnesemia, or seriously low levels of magnesium in blood. 