Younis, H. H., A. H. Alzahrani, and A. S. Alomar. “Association Between Age at Onset of Multiple Sclerosis and Vitamin D Level in Saudi Population”. International Journal Of Community Medicine And Public Health, vol. 6, no. 9, Aug. 2019, pp. 3728-31, doi:10.18203/2394-6040.ijcmph20193960.