Does the sensation of a drop of urine during prayer, with doubt about its nature and its foul smell, invalidate ablution? And what is this substance?

A drop discharged from the private parts nullifies ablution (Wudu’), whether it is urine, discharge, or water from Istinja’ (washing after relieving oneself), because it exits from one of the two orifices. However, if it is sweat, it does not nullify ablution unless it exits from one of the two orifices, which is inconceivable. The exit of such a drop during prayer invalidates it and necessitates its repetition.