I don't know if this is caused by a vitamin deficiency or not. I've been searching the Internet and nail problems can be caused by fungus, yeast, dreadful diseases - you name it! My big toenail (just one) is yellow and has a horizontal line running through it. I've been trying to "cure" it for 2 years. The first year I used a cotton ball to put vinegar on it twice a day. The second year I used a q-tip to put tea tree oil on it twice a day. It always looks "good" below the line, but as the old line grows out, a new line grows in and it looks bad above the "new" line. Kind of hard to explain. I did find a website that said yellow nails are often caused by diseases of the lungs, thyroid, or lymphatic system. But I would think some serious disease would affect more than 1 nail! (I hope!). Oh yes, another website said it could be caused by malnutrition. Again, I would think it would affect all nails? Any suggestions or am I in the wrong place? THANKS!