Nick Zito, the New York-based trainer who on Monday accepted a 15-day suspension for a positive test for clenbuterol in one of his horses, said Tuesday that he was unable to explain how the positive arose and that it may have been a mix-up at his barn. Zito said that he was in Florida on Feb. 24, 2012, when a horse he trained, Alvito, won the fifth race at Aqueduct and subsequently tested positive for clenbuterol, a bronchial dilator. His longtime assistant, Ricardo Troncozo, told him that the horse had not been treated with the drug, although another horse in the barn was given the medication earlier that week. “Ricardo swore up and down that he has no idea how that occurred,” Zito said. “Under the rules, I’m the absolute insurer, and so I thought it was best to accept it.” Zito was last suspended for a positive test in 2003, when he accepted a 10-day suspension for a positive for lidocaine, a local anesthetic. The horse tested positive in 2001, but Zito spent two years trying to get the result thrown out. “We’ve operated our whole lives the right way,” Zito said. “We’re proud of what we do. And we are fans of the rules. The rules protect me.” Zito said that in the Alvito case, his lawyers subjected the split of the horse’s postrace samples to a variety of tests in order to determine if the initial finding was correct. As a result of the time it took to conduct the testing, Zito said, it took more than a year for him to decide to accept the penalty.