I have read some of the comments where people refer to Lab time that is mandatory. Does lab mean the OR or does it mean Lab like with pipetes and petri dishes?
It usually means basic science research (i.e. "pipettes", etc.).
However, some programs allow you to do other things (PhD, MPH, clinical research) during your "lab years", so it never hurts to ask during the interview process. Some programs are more flexible than others.