There are several conditions to consider for localized dark skin at the back: first, consider the possibility of lichen planus, which is mostly seen in areas that sweat a lot in summer, such as the chest and back and underarms. It is mainly related to fungal infection due to local humidity and heat, and can be treated with topical antifungal drugs such as bifenazole cream. If the condition is serious, you can systematically use antifungal drugs, such as itraconazole capsules. Secondly, consider the possibility of coffee spot, the cause of which is mostly related to genetic factors. It usually does not require treatment, and those who feel that it affects the aesthetics can choose laser treatment. If there are multiple coffee spots on the body, the possibility of neurofibromatosis should be excluded. Finally, it is considered to be post-inflammatory hyperpigmentation, which is mainly caused by the decrease of sulfhydryl group in the skin due to inflammation and the increase of complexinase activity, and can be treated with topical hydroquinone cream, etc.