You're going to hear a lot of differing opinions here, but your main options are a purse, tote/briefcase, or a zippered padfolio. Honestly, I think you'll be fine with any of them, but I'd recommend a tote over a briefcase.
If you go with a purse or tote (I define a tote as a bag that holds letter-sized notebooks or a laptop, and would not be carried as an everyday bag otherwise), make sure you've cleaned out the non-essentials so you don't have to dig through junk in your bag in your interview.
I prefer a tote and then carry a thin folio in it, but to each her own.
EDIT: To expound upon this opinion: in your interview day, they will most likely give you a packet of information. You should also have a packet of important documents such as your personal statement, AMCAS activities, etc. If you are holding a purse in one hand and a folio in the other, it is more difficult to shake hands with your interviewers as you enter/leave. If you can get away without a purse by just carrying a zippered folio, or if you're like me and have to have a bag, then if you carry a tote in which to put the packet, then it saves you the awkward hand switch / stuff adjustment phase. It's also one less thing to carry on tours. Silly reason? Maybe. But that's my thought process
🙂
(FYI, this is my tote -- in black, not red, and they did a horrible photoshop job re-coloring the picture on the front page...the "more photos" link is better)
http://www.levenger.com/PAGETEMPLAT...Category=11-1001-1002|Level=2-3-4|pageid=6578
Regardless, the bag should match your shoes in color, and be leather, faux leather, or quality microfiber.