Is ghusl obligatory for a woman who suffers from waswas (obsessive doubts) if she doubts whether water reached behind her knee during ghusl, knowing that the area is defiled with urine and she believes that the water will reach it to remove the impurity?

It is sufficient for ghusl (ritual bath) to ensure that water covers the entire body, with the preponderant belief that it has reached everywhere, while paying attention to the inside of the leg if the ghusl is performed while sitting. It is not a condition to remove impurities before ghusl, as the water of ghusl purifies them. To treat waswas (whispers/obsessive thoughts), one must completely abandon and ignore it, and not give in to it.