What if at birth physical examination led her parents and doctors to conclude she was female, because she didn't have a penis or testes? What if her anatomy is such that sincere, honest people would say she is neither clearly male nor female, but probably more female than male?
There's 7billionish people on planet earth. Caster is a very rare scenario, it seems. Maybe she should not compete in the women's division, but that conclusion shouldn't be reached without serious thought and even some confusion, IMO.