1. School of Basic Medical Sciences, Ningxia Medical University, Yinchuan, Ningxia 750004, P.R. China
2. School of Public Health and Management, Ningxia Medical University, Yinchuan, Ningxia 750004, P.R. China
3. Department of General Practice, General Hospital of Ningxia Medical University, Yinchuan, Ningxia 750004, P.R. China