Final answer:
The most likely diagnosis for the symptoms described is a fibroadenoma, which is a benign and mobile breast tumor common in young women. Breast cancer typically presents with fixed masses, and mastitis is associated with infection symptoms.
Step-by-step explanation:
The most likely diagnosis for a single, rubbery, mobile breast mass in a 21-year-old female is fibroadenoma. Fibroadenomas are benign breast tumors that are most common in young women. They are typically round, firm, and easily movable lumps. In contrast, breast cancer lumps are often irregular in shape and can be fixed or immobile. Mastitis, an infection of the breast tissue, usually presents with redness, swelling, and pain rather than a mobile mass. A cyst is a fluid-filled sac that can also be found in the breast, but they are often soft and can fluctuate in size with the menstrual cycle. It is important for women to perform a monthly breast self-exam because it can help detect lumps or other changes in the breasts early, which is crucial as breast cancer prognosis is better when caught at an earlier stage. Breast cancer is indeed the most common type of cancer in females, and although rare in teens, it becomes more common as women age.