No one has yet mentioned that if you are even considering medicine you need not only good grades but voluntary work (long term volunteering is seen as showing more commitment) and work experience. Far better to spend time on that and find out if medicine is really for you than do a 5th A level. The only good reason to do 5 AS is if you haven't decided what you want to do and need the 5th to keep your options open. Doing 5 at A2 is unnecessary.
Medical schools demand work experience because it's a long and demanding course and not one to enter lightly. You'll be studying for the rest of your life, the hours will be long, the pay not great compared to other jobs for graduates with excellent academic results and good people skills. It's intensely competitive with 60% of applicants getting no offers first time round. It continues to be competitive with most people becoming gps when some of them hoped to be surgeons. It's not that unusual to fail one or more medical school exams and need to resit at least the exam and sometimes the year. It's not a family friendly career if you want to be in hospital medicine.