On my buttocks there is a line of grayish skin circling the cheeks. It seems to be where skin touches skin. I wash religiously so it isn't a hygiene problem. I don't think so at least. It doesn't itch or cause me any discomfort. I haven't had sex with anyone. More than anything it is a cosmetic problem as it is rather unsightly. I'd really like to know what causes it and what I can do to change it.
The only diagnosis I can think of is fungus, which can look like what you describe. This has nothing to do with dirt, as you say. Try oral clotrimazole 1% (over the counter) and make an appointment with a doctor in case it doesn't go away. There are lots of prescription-strength creams which should take care of it.
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