Invasive Ductal Carcinoma Breast

A 54-year-old postmenopausal woman presents with a screening-detected spiculated mass and microcalcifications in the left upper outer quadrant. Core needle biopsy shows infiltrating cords, nests, and tubules of pleomorphic malignant epithelial cells with vesicular nuclei, prominent nucleoli, mitotic activity, and a dense desmoplastic stromal reaction. Immunohistochemistry reveals ER 95% strong nuclear staining, PR 80% positive, HER2 3+ membranous overexpression (ISH amplification ratio 6.2), and Ki-67 35%. The structure marked **A** in the diagram represents the histologic diagnosis. Which of the following best characterizes the defining microscopic feature that distinguishes this lesion from invasive lobular carcinoma?