What is the best diagnostic step after an initial ultrasound for a woman with a stable breast mass?

The best diagnostic step following an initial ultrasound for a woman with a stable breast mass is aspiration of the dominant mass. In cases where a breast mass is detected, ultrasound provides valuable information about the characteristics of the mass, such as its size, shape, and whether it appears solid or cystic. Aspiration allows for the extraction of fluid from the mass if it is cystic or the collection of cells for cytologic analysis if it is solid. This can help determine whether the mass is benign or requires further investigation.

Aspiration is a less invasive procedure than other options and can provide immediate results in many cases, particularly if the mass is fluid-filled. If the aspiration yields clear fluid and the mass resolves, further intervention may not be needed. In contrast, excisional biopsy, mammography, and magnetic resonance imaging are generally reserved for more complex cases or when the characteristics observed on ultrasound warrant a more comprehensive evaluation.

Thus, aspiration of the dominant mass is the most appropriate first step to further evaluate the nature of the breast mass after an initial ultrasound shows a stable condition. This approach helps to minimize unnecessary surgical interventions and focuses on confirming the diagnosis promptly.