Do a practice B/B section. I'd say a prep book + khan academy is probably your best bet since it will focus on only the relevant content. Taking an actual class on mammalian physio will likely include things you don't need and miss stuff you do. The MCAT is really testing how well you can prepare for the MCAT, not how well you understand mammalian physio, so taking an actual class isn't going to be as helpful as just focusing on test prep.