What is the most likely diagnosis for a woman with a firm, mobile breast nodule and a mammogram indicating a well-circumscribed mass?

The most likely diagnosis for a woman with a firm, mobile breast nodule and a mammogram indicating a well-circumscribed mass is a fibroadenoma. This condition commonly presents in younger women and is characterized by a discrete, palpable mass that feels firm and mobile upon examination.

Fibroadenomas are benign tumors made up of both glandular and stromal tissue and typically present as well-circumscribed, solid masses on imaging studies. The description of a well-circumscribed mass on a mammogram aligns with the typical characteristics of a fibroadenoma, as they tend to have smooth, defined edges, making them distinct from more aggressive lesions.

In contrast, other options such as cystadenoma and invasive carcinoma would present differently. Cystadenomas are not typical breast pathologies and would be more aligned with ovarian masses. Invasive carcinoma typically presents as an irregular or spiculated mass and can exhibit lymphadenopathy or other concerning features, rather than the well-circumscribed nature described. Phyllodes tumors, while they can also present as palpable breast masses, are usually larger, fast-growing, and can present with more aggressive features compared to fibroadenomas.

Thus, the features of the nodule described