What is the ruling on the prayer of one who felt a drop of urine exit during prayer due to kidney disease, and must he make up this prayer?

The sensation of urine exiting does not nullify ablution (wudu') unless there is certainty of its exit. This is because the default state is purity, and this default state is not invalidated by doubt. Certainty is not removed by doubt, as evidenced by the Prophet's (peace be upon him) saying: "If one of you finds something in his belly and is unsure whether something has exited or not, he should not leave the mosque until he hears a sound or smells an odor."

However, if you are certain that urine has exited, then ablution is nullified, and you are required to repeat the prayer. This is not considered incontinence (sals) unless it is continuous for most of the time.