Just tell the truth! Your personal statement should already contain the bulk of it but now you have time to expand on it and add little things you couldn't fit into the essay. You can also add in things that have happened since you wrote the statement - for example, maybe you wrote about how your interest started when a pharmacist helped your family member, but now that you are in organic chem II and/or got a job in a pharmacy you are even more convinced that this is the career for you.