Image IQ: 47-year-old with Irregular Mass in Dense Breast Tissue

A 47-year-old woman presents for screening mammography and requests a screening breast ultrasound. On mammography, no suspicious masses or areas of calcifications are identified. The breast tissue is extremely dense.

On ultrasound, there is a newly identified irregular hypoechoic mass in the left 2:00 area. 

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What is your diagnosis?