The hallmark symptom of a breast abscess is a painful, erythematous lump because the abscess represents a localized collection of pus due to infection. This leads to inflammation, swelling, redness, and tenderness in the affected tissue. Pain and erythema are key indicators distinguishing an abscess from other breast lumps, which may be painless or present differently. Early recognition facilitates timely surgical drainage and antibiotic therapy, preventing progression and complications.

Reference: Sabiston Textbook of Surgery, 21st Edition.