Your arts job will not help. Try to find a job or internship with something closer to your interest in medicine.
I am sure this answer is well intended, but it's just plain wrong. It's a common misconception that your extracurricular activities and work experience need to match your intended major. But it's a bunch of nonsense.
For one thing, most colleges don't give a hang about your extracurricular activities anyway. There are over 2500 four-year colleges and universities in the U.S., and most of them will admit you if you're academically qualified. Only the selective ones care at all about extracurricular activities, including work experience. And you see, these colleges know something very important: at least half of undergraduates end up taking a bachelor's degree in a field different from the one they expressed interest in when they applied. This makes sense, especially at big, prestigious universities such as Penn that are filled with remarkable professors in fields of study that high-school students never even encounter. So, sure you can tell Penn what you think you want to study, and you can write an essay about your "passion" for whatever (I swear, if I read application essays, I would automatically reject any applicant who used the word "passion"!), but you don't need to tailor your work experience to it.
If you like the job in the arts center, and you do it well, do the job in the arts center. And do it as well as you possibly can.