Skin that darkens in patches can have more than one reason. Readers in Chelsea, Michigan often want to know where to start. This page covers what two published sources say, and a page on postinflammatory hyperpigmentation covers one specific cause.
More melanin in the skin
medlineplus.gov explains the basic mechanism: "Darker areas of skin (or an area that tans more easily) occurs when you have more melanin or overactive melanocytes." In short, darker skin areas trace back to melanin, and melanocytes are the cells involved.
Medications on the list
my.clevelandclinic.org includes this among its list: "Medications, such as oral contraceptives (birth control pills) and drugs that cause sensitivity to light." Check your own medication list when a patch appears. For the whole list, read the overview of hyperpigmentation it comes from.
One patch, more than one possible reason
Because there can be several reasons, a patch deserves a proper look. Bring a list of what you take and when you first noticed the change.
Naming the cause before pricing a plan
Treat a plan priced before the cause is known as a ballpark. Ask for a firm quote after the exam. If the pattern looks like melasma, say so at that visit.