what is the dark skin in folds caused by
What is acanthosis nigricans? Acanthosis nigricans is a type of skin condition characterized by thick dark (hyperpigmented) skin which tends to occur on the skin folds â armpits (axillae), neck, and groin. Typically the hyperpigmented skin has a velvety texture and can therefore be clearly differentiated from hyperpigented areas of normal skin.