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.
References Acanthosis Nigricans 28613711 
NIH
Acanthosis nigricans is a cutaneous manifestation of an underlying condition. It usually develops in skin folds, such as the back of the neck, axilla, and groin, where it presents as velvety hyper-pigmented patches with poorly defined borders. Acanthosis nigricans is most commonly associated with diabetes and insulin resistance, but rarely it can be a sign of internal malignancy. It can also occur with hormone disorders or with the use of certain medications like systemic glucocorticoids and oral contraceptives.
 Current treatment options for acanthosis nigricans 30122971 
NIH
Acanthosis nigricans (AN) is a common dermatologic manifestation of systemic disease that is associated with insulin resistance, diabetes mellitus, obesity, internal malignancy, endocrine disorders, and drug reactions. Treatment of AN primarily focuses on resolution of the underlying disease processes causing the velvety, hyperpigmented, hyperkeratotic plaques found on the skin. Initial considerations for the AN workup include evaluating patients for insulin resistance syndrome characterized by obesity, dyslipidemia, hypertension, and diabetes mellitus type II. For cosmetic treatment, topical retinoids are considered the first-line therapy for insulin-resistant AN by modifying keratinization rate. However, topical tretinoin requires application for long durations and improves hyperkeratosis, but not hyperpigmentation. Topical salicylic acid, podophyllin, urea, and calcipotriol also require frequent application, while TCA peels may provide a faster and less time-intense burden.