While not every job will allow you to choose the type of reports you prepare, some larger medical facilities/organizations have medical transcriptionists that work within each specialty area. If you are familiar with a certain area of medicine, being able to understand what you are typing often makes for more accurate reports, as a transcriptionist can pick up any errors or the need to modify the structure of a sentence. If you have a preference, share it with the interviewer. Remember, though, to mention that you are willing to work where you are needed.