Yellow vertical lines on the left big toe may be a physiological phenomenon, or it may be caused by certain diseases, such as nail fungus disease, trauma, and so on. 1. Physiological phenomenon: Some individuals have yellow vertical lines on the left big toe, which is a normal physiological phenomenon and is usually not accompanied by other symptoms. 2. Nail fungal disease: Nail fungal disease is usually caused by fungal infection of the nail plate, resulting in discoloration of the toenail, hypertrophy, vertical lines on the toenail, etc. are the main symptoms of this disease, and therefore may appear as yellow vertical lines on the left big toe. 3. Trauma: the toenail suffers from external impact, resulting in the rupture of capillaries under the nail plate, the toenail will sometimes have a vertical stripe in the late stage, with the development of time, the vertical stripe will slowly disappear, this situation usually does not require special treatment. There may be other reasons for the yellow vertical line on the left big toe. If accompanied by other uncomfortable symptoms, it is recommended to go to the hospital in time for examination, to clarify the cause of the disease, and then take corresponding treatment measures under the guidance of a professional doctor, so as to avoid delays in the condition.