Take note that the answer field is simply an input form field. You can type anything in there. I happened to type 'Duck' for my answer to make this point.
As expected, my answer was wrong. It should have been 'substantial.' What if I had put in 'substntial' (missing the first 'a'?). The answer would still be wrong.
And that is because there is no constraint on what the input might be. A multiple choice answer would have been much better, allowing the user to at least have a clue as to what the answer might be, and then not getting frustrated (if you can get frustrated with such a simple game) by a simple typo.
But wait, you say, "Maybe the IRS wants to demonstrate that you need to know the answer by heart? And no hints allowed, other than through the question." I get the point. I think it's a stretch in any web-interface situation like this, but I get it. In that case, if it is that vital, the answer system should accommodate common spelling errors. Google does it all the time; as do auto-spell checkers in word processing apps.
In the case of this app, it wouldn't be too hard; there are only a few options total in the entire game and a quick check wouldn't detract from the interface anymore (than the interface detracts from itself; sorry, IRS web dev folks; you obviously weren't given a very big budget).
So recognize the errors that a human being might incur through mis-typing. Put constraints on options in order to reduce the amount of frustration your user will encounter because of simple spelling errors and the like.

