One of the great things about a career in healthcare is that there are so many options. Having ever considered a career shift into a specialty area is not something that will disqualify you from being hired as a family practice physician. This question really just creates an opportunity for the interviewer to explore what areas are of interest to you. If you have considered a more concentrated specialty, share that with the interviewer, but end your answer with why you have stayed with family practice so far.
"I did consider a career as a pediatrician at one time. However, when I began to explore options, I felt like family practice would give me an opportunity to work with a more diverse group of people. The decision to focus on family medicine has been a great fit for me."
"Actually, I considered several different specialty concentrations. I thought about obstetrics and urology. I also even considered cardiology. For me, though, I really felt like general practice was a good fit and that it would give me the opportunity to take care of a broader group of patients."
"I never considered any other specialty than family medicine. I respect concentrated specialty areas and have some close friends who are specialist. It takes all of us to make a good healthcare network, and I'm happy to be a small part of it."