What is the ruling on one's undergarment becoming defiled by urine when not immediately purified, and does this affect the validity of past prayers, and must they be made up?

If your undergarment is soiled with urine and you are certain of it, then the soiled area must be purified, or the doubted area must be washed if you are unsure.

If you prayed with a ritually impure garment out of ignorance or forgetfulness, your prayers are valid. However, if you intentionally prayed with it without an excuse, then you must make up all the prayers you performed with that garment due to the absence of the condition of purity from filth (khabath).

If you cannot determine the number of invalid prayers, you must continue to make them up until you are reasonably certain that you have fulfilled your obligation.

If you are unsure whether your garment has been soiled with impurity, you are not obligated to do anything, and your prayers are valid according to most scholars, contrary to the Malikis who require sprinkling the garment with water.