Is it obligatory to wash the entire limb or only the part about which there is doubt when one is uncertain about washing a part of the limb during ablution, after having finished washing it?

A person afflicted with obsessive doubts (waswas) who experiences frequent doubts about his ablution (wudu'), such that it occurs daily, even if only once, should disregard these doubts and not act upon what he suspects he omitted. As for a person not afflicted with obsessive doubts, if he doubts during ablution that he missed washing part of a limb, he should wash only that part he doubted, and then wash the subsequent limbs of ablution in their proper order.