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It must be schambergs disease ,which manifests as dark black or brown spots over fette and ankle .They are asymptomatic and are benign.
They occur because of breakage of small blood vessels(capillaries) present around the feet .The broken down hemoglobin
gets deposited underneath the skin and appears as dark spots.This breakage is because of pressure from the whole body or like weight lifting or also seen in football players.
You should worry mush about this(except for cosmetic reasons) ,as its just a benign condition .
Treatment is also limited .You can use tretinoin
cream locally ,and if that doesn't help then you can go in for lasers.Lasers would be a very costly affair for such a benign condition.
You should meet a dermatologist
to confirm the diagnosis.
I hope this answers your query,if you have any more questions you can write back to us.