Acanthosis Nigricans (Neck)

In people with acanthosis nigricans, skin in the armpits, on the nape of the neck (top), and in other folds of skin may become dark and thick. In people with dark skin, the skin may have a leathery appearance (bottom).

(Acanthosis nigricans is most often a skin manifestation of type 2 diabetes, but it may also mean a person has internal cancer, especially if it started suddenly and spread widely.)