Dark patches in areas like underarms, knuckles, and joints can also be associated with acanthosis nigricans or other conditions such as: 1. Friction:  Darkening of these areas can occur due to friction or rubbing, particularly if there's repeated friction from clothing or activities. 2. Hyperpigmentation:  Increased pigmentation in these areas can result from excess melanin production, which can be triggered by factors like sun exposure, hormonal changes, or inflammation. 3. Post-inflammatory hyperpigmentation:  Previous irritation or inflammation in these areas can lead to darkening as a response to the healing process. 4. Skin disorders:  Certain skin conditions like eczema or psoriasis can cause darkening of the skin in these areas due to inflammation and pigmentation changes. 5. Hormonal changes:  Fluctuations in hormone levels, such as those occurring during pregnancy or as a result of hormonal disorders, can contribute to dark patches on the skin. It...