The clinical presentation—tender, swollen breast with erythema and fever—in a lactating female strongly suggests acute mastitis. The sonographic findings support this diagnosis. In the image, the breast parenchyma shows diffuse, hypoechoic, and heterogeneous echotexture with increased vascularity, which is consistent with inflammatory changes typical of mastitis.

Mastitis is a common complication during lactation, particularly in the first few weeks postpartum. It results from milk stasis and subsequent bacterial infection, commonly due to Staphylococcus aureus. Ultrasound features of mastitis include:

Ill-defined, hypoechoic, edematous areas in the breast parenchyma

Increased Doppler flow due to hyperemia

Skin thickening

Ductal dilatation may also be present

If left untreated, mastitis may progress to abscess formation, which would appear as a localized, complex fluid collection with peripheral hyperemia and internal debris. However, the image does not show a well-formed fluid collection consistent with abscess.

Option B (Ductal carcinoma): Inappropriate here due to the acute clinical scenario and patient age. Ductal carcinoma typically presents as a hypoechoic mass with irregular margins and posterior shadowing, not diffuse edema or inflammatory changes.

Option D (Galactocele): This benign milk-filled retention cyst typically appears anechoic or with fluid–fluid levels but lacks signs of inflammation and systemic symptoms such as fever.

Option A (Abscess): This could be a differential, but abscesses usually present with a well-defined anechoic or complex mass. The absence of a discrete collection and the diffuse appearance makes mastitis more likely.