Drawing A Blank

I always draw blank at the worst times. It usually happens when I’m taking a  multiple choice test and I come across a question I know. But when I look at the the answer choice I lose focus and forget what would be the reasonable answer. Then when I choose an answer, I second guess myself. After drawing a blank on one question I begin to draw a blank on the rest of the questions. My train of thougth goes spiraling down until, I finish my test. The next second I turned in the test everything comes back to me.