How to know the cause of urticaria in one year old baby? treatment? and could it result in hypomelanosis?

Urticaria: In a one year old is uncommon,especially chronic types, even though,most of the time the cause would be unknown & would resolve spontaneously,just get controlled with medicine like cetirizine.Allergies to certain foods can be a cause,but mostly it's obvious with direct correlation with the offending food as milk or peanuts.Right diagnosis leads to right management,check again,have a 2nd opinion