What is the ruling on one who feels a drop of urine exit after istinja and wudu, and must the garment affected by it be washed, and is prayer in it valid?

If you are certain that a drop has come out, then you must perform Istinja' (washing the private parts) and Wudu' (ablution) for every prayer, and wash whatever parts of your clothes it has touched. However, if you are in doubt, then there is nothing upon you, and you should ignore such doubts to avoid falling into Waswasa (whispers/obsessive thoughts).