Sepeng, who was not available to comment, has until Monday to explain to the ASA how the substance came to be in his system and whether or not he wants his 'B' sample to be tested.
Norandrosterone is the metabolite of the steroid nandrolone, which shows up in a urine sample.
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"There have been hundreds of cases where athletes have escaped a ban because nandrolone has been found in contaminated supplements," he told Reuters.
Springbok rugby player Cobus Visagie won a case in 2001 when he proved his body naturally produced excessive levels of nandrolone under stressful conditions.