It is fair to say that UTSW is a research school. However, even primary care has a lot of research which drives changes in practice of medicine every year.
You never go into interviews trying to give different answers to different schools. You can be interested in any field of medicine at the time of the interview but you should have done basic research on the attributes of the school for what you are interested in. The school wants to know how you are going to use the resources made available to you in whatever area you are interested in (you get about 6-12 months where you can do whatever you want at UTSW) and you should be prepared to state that. So if you are interested in primary care, you should know what free clinics UTSW gives you access to where you will go volunteer and state that during the interview. They will ask pointed questions about what you state in your primary and secondary and your activities and to see how you are able to extend any medical relevant activities while at UTSW. You should be able to articulate a vision where you will be at UTSW volunteering at a clinic, researching in a specific field with someone specific based on what you have already done in that area or even something new you want to develop an interest in.
If I were interviewing at UTSW, I would want to know how to get involved in their latest research grant related to nutrition. 1/3rd of the people in the country being labelled obese so it is pretty much primary care at this point.