The fingernails can turn every which color depending on what is going on in the body. I didn't find much on the color brown. It could be a fungal infection. I found one reference to having too much flouride in the body but wasn't sure that applied either since it didn't mention a line across the fingernails but rather seemed to say the entire fingernail would discolor. Unfortunately, I couldn't find a single photo that looks like what you are describing so not sure. May sound like a dumb question but you haven't gotten your hand caught in a door lately? Possible bruising?
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