Dear Mithun, 

Could you please clarify if 53% is in the first year of MBBS only, or the aggregate percentage of the complete 5.5 years course. If its final aggregate and you belong to OBC/General category, then I am afraid you are not eligible, otherwise if its only of first year, then its alright. You can build upon your percentage in coming years by performing better. 

As per the eligibility criteria of AIIMS PG 2018, the aggregate percentage required at MBBS level is 55% for unreserved and OBC category, while its 50% for SC/ST. 

Hope it clears. All the best!