Many things: Acanthosis nigricans causes dark, velvety patches of skin on the neck & skin folds and since it can be associated with diabetes or other health issues, it is important to have it looked at. Many other causes exist: sun-exposure, post-inflammatory hyperpigmentation, tinea versicolor, other rashes & even retained dirt (terra firma-forme) are just a few! see a dermatologist.
Answered 10/13/2012
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