Does water entering the anus while fasting, without being certain that all of it has exited, invalidate the fast?

The invalidation of fasting by introducing water into the anus is a matter of disagreement among jurists, but the majority of scholars believe that fasting is invalidated by it. If you do that, your fast is invalidated, unless you are truly compelled, in which case you may adopt the view of those who do not consider the fast to be invalidated by it.