It's hard to say what they're really after with the writing sample. I've asked UCSF students what they thought the purpose was, their guess was as good as anyone else?s. It is most likely used to make sure you can communicate adequately in English in written format, and to see if you have the ability to put two thoughts together. If you are good at making a strong case quickly then the writing sample shouldn?t be a something to stress about. However, if you are a poor writer like myself you may want to do a little prep beforehand.
My advice to you if you feel uncomfortable about your writing skills is to quickly locate a Kaplan MCAT book and practice their easy method for writing a slamming essay for the MCAT essay portion of the exam. It teaches you how to QUICKLY organize you thoughts and produce a coherent and rapid argument for most essay questions. At my UCSF interview my question was similar in tone and complexity to MCAT essay questions I practiced before the interview. It asked about Supreme Court justices and why they should or why they should not have life time appointments for their positions. For me it was a difficult question because I honestly have very little background or interest in politics or government (I slept through all my government classes). But, I used what Kaplan taught me, made up an argument and supported my argument with examples (albeit weak ones), but apparently it did the trick.
Have fun at your interview. They are extremely nice and treat you well.
