Hello

Dear, taking drop is not so easy. It requires  determination to go for it. If you are determined to put complete efforts for one complete year and give your Bestt in the preparation for the entrance.. then just go for it.
Sit with your parents and ask them to give you time to explain them. Then discuss about your interest that what you want to do.. tell them your preparation method and explain this way. There are many students who take drop for 2-3 years and then start their Medical career. Not everyone gets success at first.. it takes time. Keep all your points this way in front of them and assure them that you'll devote yourself completely to achieve your target.
Most importantly.. stay calm and normal .. explain them calmly coz they don't have idea regarding this field and related preparation n stuff.. and so it's your duty to make them clear about education plans. Hope you're getting my point. !

Hope this helps.!
Do not worry and be positive.

Good Luck.!