If there were one reason not to accept you, what would it be?
When a faculty member asks you this tough question in an interview, her motivation might be to determine whether there is a weakness in your application that she is missing. Or she may be assessing how you manage stressful situations by posing a question that is unpleasant.
While you need to be honest throughout the entire application process, you do not need to volunteer information that might harm you.
You can say, “While every candidacy has room to improve, I think I have a strong application." Then you can use the question as an opportunity to mention the strengths of your candidacy.
Contact me for Mock Interviews. I still have January slots open as of this writing.