My sister does that all the time to her students. When they catch on she tells them that the one question with a different letter answer will be worth 50% of the test grade.
Hold on a sec this is actually a really shitty thing to do. Do you realise how fucking stressful exams are for students, knowing that their future is relying on how well they do is one test? This sort of shit doesn't fucking help.
One of my teachers gave a test every year where every answer was a for 30 some years. I just went with it and didn't care that all my answers were a. I ended up being the only student in his entire teaching career (he's retired now) to ever get a perfect on that test
She got honors.