My sister has flat brown spots the size of silver dollars on her back, grouped in clusters. She also has a dark line running along the center of her back. Does this sound dangerous?

— Bern, Texas

If your sister has had the brown spots and dark line on her back since childhood, they could be a type of birthmark called café au lait spots. The spots themselves are usually harmless, especially if there are only a few of them. However, if there are six or more, and if they are associated with freckles in the armpits or groin and lumpy growths on the skin, they could be a sign of neurofibromatosis, a genetic disorder that causes abnormal growth of nerve tissues in the skin, bones, and eyes. While genetic testing for this condition is available, most cases can be diagnosed by a doctor based on a physical examination and eye exam.

If the brown spots are new, tinea versicolor could be the cause. This is a common, superficial yeast infection that often occurs in warm, humid climates or with increased perspiration and typically affects the upper chest and back. "Versicolor" refers to the production of different-colored spots; some people may develop brown spots, while others can have white or pink patches. A dermatologist can diagnose tinea versicolor by gently scraping the surface of the brown spots and looking at the skin cells under the microscope. If there are only a few spots, they usually respond to over-the-counter creams such as Lotrimin or shampoos like Nizoral 1 percent or Selsun Blue; if there are many spots covering a large area, however, they may require prescription-strength topical creams or antifungal pills such as Nizoral tablets.

Because any dark or discolored growths should be brought to a doctor's attention, I suggest that your sister visit a dermatologist, who can diagnose her condition and recommend the best treatment.

Learn more in the Everyday Health Skin and Beauty Center.