By Iain Axon, Karolos Grohmann and Joseph Campbell BEIJING (Reuters) -Russian figure skater Kamila Valieva has argued that her positive drug test was caused by a mix-up with her grandfather’s heart medication, an Olympic official said on Tuesday. The 15-year-old’s defence was revealed as Beijing braced for an unprecedented Olympic moment – the world’s top figure skaters will compete in the evening’s single competition with the likelihood they will not receive medals at the Winter Games. Denis Oswald, the International Olympic Committee’s permanent chair of the disciplinary commission, said Val…