A 36-year-old woman comes to the office after she noticed some lumps under her arm while getting dressed. She says she feels fine and has no other symptoms. Her medical history is significant for hypothyroidism. Family history is negative. On physical examination, her body mass index is 24 kg/m². Breast examination shows nontender left axillary lymphadenopathy. The patient is sent for an ultrasound-guided biopsy. Microscopy of the tissue specimen shows clumps of cells that have positive immunohistochemical staining for cytokeratin. These cells most likely belong to which of the following lineages?

Question

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Cytokeratins are proteins that help form the keratin-containing intermediate filaments that make up the cytoskeleton of almost all epithelial cells. Keratin proteins are highly abundant in exposed stratified squamous epithelium such as the epidermis and oral mucosa and are less abundant but still present in simple epithelium such as that found in the liver, gut, and pancreas. As a result, cytokeratin is a commonly used immunohistochemical marker for epithelial-derived tumors, such as breast cancer. Keratin is also the major protein component of hair and nails. Keratin is composed of large amounts of alanine and glycine that, because of their small structure, are able to coil tightly and form large numbers of hydrogen bonds. In addition, keratin contains a significant amount of the sulfur-containing amino acid cysteine, and formation of disulfide bonds between cysteine residues gives additional rigidity and toughness to the protein. (Choice B) CD20 is a marker used to identify B lymphocytes, and CD3 is used to identify T lymphocytes. (Choice C) Vimentin is an intermediate filament present in mesenchymal tissue and can be used to detect sarcomas. (Choice D) Myeloperoxidase is a peroxidase enzyme that can be used as a marker for myeloid cells such as those in acute myeloid leukemia. (Choice E) Chromogranin A and synaptophysin are markers used for neuroendocrine tumors. Educational objective: Cytokeratin is a commonly used immunohistochemical marker of epithelial cell carcinomas. __________ References: Recommendations for human epidermal growth factor receptor 2 testing in breast cancer: American Society of Clinical Oncology/College of American Pathologists clinical practice guideline update. (http://www.ncbi.nlm.nih.gov/pubmed/24101045)

A 36-Year-Old Woman Comes to the Office After She Noticed