What is meant by something exiting the anus that nullifies wudu? Is it what reaches the underwear or what exits directly from the anal opening, taking into consideration the frequent exit of small or large amounts of stool after purification due to irritable bowel syndrome?

The exit of feces from the anus invalidates ablution (wudu') as soon as it reaches the outer part of the exit, even if it does not spread or flow. If this is not continuous for most or all of the time, it does not fall under the ruling of incontinence (sals).

According to the majority of scholars, prayers performed with ablution invalidated by such an exit must be made up, even if the invalidating factor was unknown at the time. Sheikh al-Islam Ibn Taymiyyah, however, is of the view that an ignorant person only needs to repeat the prayer of that specific time and commit to it in the future, but does not need to make up past prayers.

However, if the discharge reaches the point of incontinence, one performs purification after the entry of the prayer time and prays, and whatever exits during their prayer does not harm them.