Correct: C

Histologic Description: This is a fairly well delineated fibroblastic nodule with extensive hyalinization. There is subtle infiltration at the edge of the lesion, and abrupt variations in cellularity, which raise suspicion for metaplastic carcinoma. The spindle cells are diffusely immunoreactive for high molecular weight cytokeratin (34BE12), supporting the diagnosis of spindle cell metaplastic carcinoma.
Differential Diagnosis: Fibroadenoma would not label for high molecular weight cytokeratin, and typically labels for CD34. Sclerosed papillomas often have a hyalinized rounded appearance, but would not demonstrate high molecular weight cytokeratin labeling. Duct ectasia is characterized by markedly dilated ducts associated with squamous metaplasia and periductal chronic inflammation and histiocytes, not seen in the current case.
This case illustrates a subtle low grade spindle cell metaplastic carcinoma. Clues to the diagnosis include abrupt transitions in cellularity within the lesion, and the layered, plaque like hyaline fibrosis within the lesion.