LOUISVILLE, Ky.—Medina Spirit’s victory in the Kentucky Derby is in serious jeopardy because of a positive postrace drug test, one that prompted Churchill Downs to suspend Hall of Fame trainer Bob Baffert immediately on Sunday in the latest scandal to plague the sport. Baffert denied all wrongdoing and promised to be fully transparent with the Kentucky Horse Racing Commission during its investigation. Baffert’s barn received word Saturday that Medina Spirit had tested positive for an excessive amount of the steroid betamethasone, which is sometimes used to treat pain and inflammation in horses. Medina Spirit’s win over Mandaloun in the Derby stands—for now. “To be clear, if the findings are upheld, Medina Spirit’s results in the Kentucky Derby will be invalidated and Mandaloun will be declared the winner,” Churchill Downs officials said in a statement shortly after Baffert held a hastily planned morning news conference outside his barn to respond to …