How can you make dark skin under your armpits, elbows, knees, and neck go away?!

How can you make dark skin under your armpits, elbows, knees, and neck go away?

My little sis has dark skin on her neck and knees she tries scrubbing it out but it doesnt come off... Any ideas on what this might be? Or how to get rid of it? serious answers please thank you!

I read somewhere this could be related to prediabetes or a thyroid problem..check that out..esp. the neck..if its not that then try the lemon juice and keep it clean, is she heavy it might be these parts are getting chafed.

Also if you've got the money you could look into a skin lightening cream for chafed or darker skin?