Final answer:
The unilateral, well-circumscribed, firm, smooth, rubbery, painless mass in the breast is most likely a Fibroadenoma. Fibroadenomas are common, benign breast tumors that typically occur in young women.
Step-by-step explanation:
The unilateral, well-circumscribed, firm, smooth, rubbery, and painless mass in the breast is most likely a Fibroadenoma.
A fibroadenoma is a common benign breast tumor that is usually found in young women. It typically presents as a single, painless lump that is round or oval-shaped. The lump is usually mobile and feels firm and rubbery.
Fibroadenomas are usually not associated with any other symptoms and do not increase the risk of developing breast cancer.