Answer
(Fatwa: 1535/1258=D/1430)
If you are sure that the urine drop was discharged and was absorbed in underwear, your wudhu broke. You will have to make fresh wudhu for performing salah and you will have to put off the underwear for salah or you can wash the affected part of the shalwar. But, if you are not sure, rather you are only doubtful and suspicious, it is sufficient to make wudhu out of cautiousness.
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband