A 25-year-old woman presents with scant nipple discharge and no palpable mass. What is the most likely diagnosis?

In this scenario, the most likely diagnosis is intraductal papilloma. This condition is characterized by the presence of a small, benign tumor within a milk duct of the breast, which can lead to symptoms such as nipple discharge, especially if the discharge is scant and there is no palpable mass. Intraductal papillomas commonly present in women in their 20s to 50s and often do not present with any prominent findings on examination aside from the discharge.

The nature of the discharge, being scant, is consistent with intraductal papilloma, as this condition typically does not lead to abundant discharge or significant alteration in the breast tissue texture. Additionally, the absence of a palpable mass further supports the diagnosis.

Other conditions can also present with nipple discharge and breast changes, but they typically have different characteristics. Fibrocystic breast disease often presents with more generalized breast tenderness or cystic changes rather than isolated discharge. Breast cancer might cause discharge as well; however, it usually presents with a palpable mass or other significant breast changes. Chest wall syndrome typically refers to musculoskeletal issues and would not account for nipple discharge at all. Thus, intraductal papilloma is the most fitting diagnosis given the details provided.