The interviewer wants you to highlight your best attribute. They are asking you what you think you are great at. You should highlight that one attribute but also give a specific example of why you think so.

"The ability to instill trust in my patients is my best asset. I can listen to patients and create treatment plans accordingly. I can also listen to physicians and take orders to improve patient care. For instance, there was an issue when I was conducting a team session - one of the patients was shy. She would barely talk mainly because she was abused as a child and thought that no one understood or cared. I went out of my way to include her in our discussions. I even encouraged her to schedule a private session with me and a physician where she was able to open up and trust me. She has now moved on to live a happier life and still lets me know how she doing from time to time."