A 28-year-old female presents for a routine physical exam. H…

A 28-year-old female presents for a routine physical exam. Her history includes infectious mononucleosis at age 18, a residence in a Northern latitude with documented Vitamin D deficiency, and a family history of autoimmune disease. Which of the following factors is considered a primary environmental trigger that activates the inflammatory process in the development of Multiple Sclerosis (MS)?