Acanthosis nigricanshttps://en.wikipedia.org/wiki/Acanthosis_nigricans
Acanthosis nigricans is a medical sign characterised by brown-to-black, poorly defined, velvety hyperpigmentation of the skin. It is usually found in body folds, such as the posterior and lateral folds of the neck, the armpits, groin, navel, forehead and other areas. It is associated with endocrine dysfunction, especially insulin resistance and hyperinsulinaemia, as seen in diabetes mellitus.

Causes
It typically occurs in individuals younger than age 40, may be genetically inherited and is associated with obesity or endocrinopathies, such as hypothyroidism, acromegaly, polycystic ovary disease, insulin-resistant diabetes or Cushing's disease.

Treatment ― OTC Drugs
#40% urea cream
  • It is common in obese people.
  • Black pigmentation and wrinkles in both armpits suggest Acanthosis nigricans.