Well, easily you could store everything in a session and you don't lose the data. You could also "lock in" that information so that when the form is presented, instead of being able to change the value, they can only click a link or button that says "Next" so they can't change their answer.
The reasons for disallowing back buttons during things like this are because if you give an answer to a previous problem in a later question, they could go back and answer again. Not really testing their knowledge, just their ability to find the answer. Other reasons are for like CC processing scripts where the user could be charged twice for the same purchase.
