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Biopsy of a lesion on the thigh of a 65-year-old man shows pagetoid cells in the epidermis. Immunohistochemical studies show these pagetoid cells to be positive for CK7 and CEA. Which of the following statements is untrue?

A. The diagnosis is most likely extramammary Paget disease (EMPD)

B. S-100 and Mart-1 stains are most likely to be positive

C. Adipophilin is unlikely to be positive

D. Another stain which might be helpful is Cam5.2

B. In EMPD, CK7, CEA, and Cam5.2 should be positive. CK20 positivity is seen less frequently, but may be seen associated with underlying gastrointestinal malignancy. The CK7 and CEA immunopositivity indicates that this is EMPD. Therefore, S-100 and Mart-1 will be expected to be negative, since they would be positive in melanoma in situ. Adipophilin labels the lipid microvesicles in sebaceous cells, and therefore this too should be negative in this case.

A biopsy shows large cells in sheets in the dermis. Immunohistochemical studies reveal that these are CD45, CD34, and myeloperoxidase (MPO) positive. The best diagnosis is:

A. Leukemia cutis

B. Cutaneous lymphoid hyperplasia

C. Marginal zone lymphoma

D. Chronic lymphocytic leukemia

A. Leukemic cells from acute myeloid leukemia may be positive for myeloperoxidase, CD34, CD117, and CD45 (leukocyte common antigen).

Which of the following may be helpful in evaluating mucin?

A. Antityrosinase antibody

B. Antitryptase antibody

C. Colloidal iron

D. Fontana-Masson

C. Colloidal iron stains mucin blue. Antibody to tyrosinase would be positive in melanocytic cells. Antitryptase antibody labels mast cells. Fontana-Masson stain is used for melanin.

A skin biopsy reveals "small blue cells." Immunohistochemical studies reveal the following profile: CK20—positive in a "dot-like" pattern; pan-cytokeratin—also positive, similar to CK20; TTF-1—negative. The diagnosis is:

A. Merkel cell carcinoma

B. B-cell lymphoma

C. Poorly differentiated carcinoma

D. Sebaceous carcinoma

A. Merkel cell carcinoma shows characteristic dot-like CK20 positivity, cytokeratin positivity, and is positive with Cam5.2 and CD56.

You are suspecting a diagnosis of calciphylaxis. Which of the following stains ...

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