The reason that interviewers mention stem cells is because it is a controversial topic (some stem cell lines involve the development and use of human embryos) that is understood at a high level by most applicants. Most interviewers will expect you to be reasonably well versed on issues of relatively common knowledge, such as the ACA and the issue you mentioned, but are not looking to probe your in depth knowledge on obscure scientific issues. Of course, you should be able to speak in depth on any issue listed in your application, such as a research project that you worked on in undergrad.