What is the most likely reason a young woman would have breast lump?

Fibroadenoma: The most common breast tumor is a fibroadenoma. This is a benign tumor which may be present as early as the teen years. As women age, breast cysts may develop, which are benign fluid-filled lumps (peak incidence ages 35-50). It is often very difficult to distinguish a lump from normal "lumpy-bumpy" tissue. When in doubt, it is best to see your primary care physician or gynecologist.

Benign mass: A fibroadenoma is a benign breast tumor that grows commonly in young women, treatment may vary because of symptoms , duration and size. However there are other causes and you should have that checked out by your obgyn or surgeon for proper work up and management.