Which initial diagnostic test is best for evaluating a mobile breast mass in a premenstrual woman?

The best initial diagnostic test for evaluating a mobile breast mass in a premenstrual woman is ultrasound. This is because ultrasound is particularly effective in differentiating between cystic and solid masses, which is crucial when assessing breast lumps. In premenstrual women, breast tissue can be more dense and changes can occur due to hormonal fluctuations, making ultrasound a preferred method for evaluation since it does not rely on tissue density to provide clear images.

Ultrasound can also help visualize the characteristics of the mass, such as its size, shape, and whether it appears solid or fluid-filled, which guides further management decisions. In cases where the mass is cystic, ultrasound can also be used for aspiration if necessary.

Other imaging modalities like mammography or MRI may be less suitable as a first-line approach in this scenario. Mammography might not clearly depict changes in dense breast tissue in younger women, and MRI is typically reserved for more complex cases or when breast cancer is suspected after preliminary evaluations. Fine needle aspiration is more invasive and usually follows imaging when further clarification of a solid mass is needed. Thus, ultrasound represents the most appropriate initial diagnostic tool in this context.