Dear Medicine or What:
First of all, we would like to wish you the very best in being admitted to medicine, in which case your question becomes mute. But we are glad that you are planning contingencies in case your most desired path does not materialize. As you have recognized, admission for medicine is fiercely competitive. Many hopes are dashed every year.
What you should do if you don't get into medicine is your decision and not ours. We can however offer you some advice. Before we begin, let us keep in mind that education is a means to an end and not an end unto itself. So the question you should ask is: What would you like to do as a career if you do not pursue medicine? The answer to this question depends on your ambitions, goals, and interests. It is on this basis that you should determine your path to your career and success.
"If I don't get into medicine, I should study the next closet thing to it" is flawed thinking. It is either medicine or not - any closeness to medicine is irrelevant because you would either be a physician or not. We recognize that this expression of our views is blunt, but it is the truth. Please consider it, even if at first you don't agree with it.
Another strategy is to temporarily studying something that is close to medicine. That is, one can apply for admission again; that additional knowledge of a field related to medicine, acquired through further education, can only help. This thinking is flawed as well to a great extent. A vast majority of candidates who re-appear for the entrance exam do not make it into medicine.
A bachelor's degree in botany, zoology, or chemistry - all close to medicine - do not offer nearly as attractive career opportunities as some other fields, say engineering, computer science, and so on. We strongly encourage you to consider all fields of interest and evaluate their potential for your future success, and not focus so strongly on their "closeness" to medicine.
We still hope that you get into medicine and wish you our best.