Which characteristic is associated with mastitis?

Mastitis is an infection of the breast tissue that results in breast pain, swelling, warmth, and often fever. The characteristic of unilateral breast swelling is particularly important, as mastitis typically affects one breast rather than both. This swelling is usually accompanied by localized redness and tenderness in the affected area. The presence of fever indicates that there is an infectious process occurring, as mastitis is commonly caused by bacteria that enter through cracked or sore nipples, especially during breastfeeding.

The other characteristics do not fit with mastitis. Regular weight gain during breastfeeding is generally a sign of effective breastfeeding and does not indicate any issues related to mastitis. Bilateral breast tenderness without signs of infection suggests conditions like engorgement or non-infectious causes, rather than mastitis. Decreased milk supply with no physical symptoms could indicate other problems, such as hormonal issues or poor breastfeeding management, but it is also not characteristic of mastitis, which is marked by specific symptoms of infection including swelling and fever.