I have horizontal ridges on only one toe--the left big toe. what might cause this? polycythemia? crohn's?

Beau's lines: Beau's lines are deep grooved lines that run from side to side on the fingernail or the toenail. They may look like indentations or ridges in the nail plate. There are several causes of Beau's lines. This may be caused by an infection or problem in the nail fold, where the nail begins to form, or it may be caused by an injury to that area. They may be a sign of an illness of the body.