Correct Answer: A. CK20
Explanation: Extramammary Paget’s Disease is an intraepithelial adenocarcinoma of apocrine glands that presents with a well-demarcated erythematous plaque in the genital area. Histologically, pagetoid cells stain positive for CK7, unlike pagetoid melanoma cells which stain positive for S100. CK20 staining has been associated with secondary EMP in the setting of underlying gastrointestinal or genitourinary carcinoma.
Reference: Ohnishi T, Watanabe S. The use of cytokeratins 7 and 20 in the diagnosis of primary and secondary extramammary Paget’s disease. Br J Dermatol. 2000;142:243-7.
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