You could probably set this up using a complex skip as described in the "Advanced Features" section of the user's guide. You'll just need to set the skipto value to the variable name of the current item if they get it wrong. Otherwise, you would set it to "none". It could also be done as a custom item--I think there is an anagram task item posted somewhere (MediaLab Sample Experiments Section?) that tests the content of the response before allowing the subject to continue. Let me know if you run into any trouble with either of these approaches.