As you might know Prometric is the provider for the technical exams for Microsoft. For MOS exams the exam provider is Certiport. You can find their website here: www.certiport.com
Certiport is not a part of Microsoft but a partner who is delivering MOS exams. If you have any questions with regards to the exams you will always be redirected to Certiport even if you ask your questions to Microsoft.
My personal experience with Certiport is excellent. They do respond very fast to any questions you might have. Bare in mind that they do not only offer Microsoft exams but also other exams for other companies. So make sure when you do mail them, you make it clear that you have a question about one of the MOS exams.