You don't need to know it word for word. It's most important to remember any buzz words, because often, graders will skim and look for the important legal terms and buzz words. So if you can formulate the rule so that the proper elements are there and the general rule is correct, then the fact that you didn't recite bar/bri's version verbatim isn't a problem.
And in fact, you can get the rule WRONG and still get points. The bar examiners do want you to know the law, and you won't get as mnay points if you get it wrong. But they also want you to be able to use logic and legal reasoning, so if you make something up that seems right, and you apply your made up rule in a way that makes sense and come up with at least a plausible answer, you'll get some credit for it (at least in most states).