During a physical examination of a client's breast, which finding would the nurse recognize as a possible indication of a malignant tumor?

A. A well-defined mass is typically benign, while malignant tumors are often irregular and less defined. B. Nipple retraction or dimpling can be a sign of a malignant tumor because cancer can cause changes in the breast's structure. C. Stretch marks are not related to malignancy and are often a result of changes in skin elasticity. D. Tenderness of the breast is more commonly associated with benign conditions, such as fibrocystic changes, rather than malignancy.