Dear Asish, say you appear for NEET and you score good and get a seat, you can show that admission letter to your current college authorities and take a TC from them. You shouldn't be cancelling your admission but ask them for a TC. With this TC you will be still pursuing your education in Medical field. Cancelling the admission shows that you are stopping your admission miday. So please ask for a TC and no one can deny you your TC no matter what the reason might be. And coming to being barred from taking admission in the medical field, unles its a disciplinary action, no one can do that to a student.