What causes the "pungent" odor of urine in clothes, and how can one purify oneself from it?

It appears that the questioner is afflicted with waswasah (obsessive whispering). Therefore, it is advised to turn away from these whisperings and not pay attention to them, for what he mentions might be mere delusions or ordinary smells. Unless there is certainty of urine having exited, nothing is required. However, if the questioner becomes absolutely certain that it is the smell of urine and that something has exited from him, then he must purify the affected area of the body and clothing, and perform istinja (definition)' (washing the private parts) and wudu' (ablution) for prayer.