USMLE Step 1, Question #664
A 41-year-old man presents to clinic complaining of vague abdominal pain and fatigue. History is remarkable for several episodes of pneumonia over the past year. Physical examination reveals marked splenomegaly. CBC shows a hemoglobin concentration of 8.2 g/dL, hematocrit of 24.6%, MCV of 90 fL, WBC count of 2400 and platelet count of 63,000.
Peripheral blood smear demonstrates many small leukocytes with kidney-shaped nuclei and pale blue cytoplasm with threadlike extensions. Immunohistochemical staining is positive for CD20.
Which of the following additional findings is most characteristic of this disease?
A. Auer rods in leukocytes
B. Monoclonal IgM spike in serum
C. Philadelphia (Ph) chromosome expression
D. Tartrate-resistant acid phosphatase in leukocytes
E. Toxic granulations in neutrophils
Yes..Right
This patient has hairy cell leukemia, an uncommon neoplastic disorder of B cells (CD20+). These cells infiltrate the spleen and marrow. Pancytopenia results from poor production of hematopoietic cells in the marrow and sequestration of the mature cells in the spleen. There are two characteristic findings with this disease: hairy projections from neoplastic leukocytes in the peripheral blood smear, and tartrate-resistant acid phosphatase in leukocytes (Choice D).
Auer rods (Choice A) are characteristic of myeloblasts in acute myelogenous leukemia (AML).
Philadelphia (Ph) chromosome expression (Choice C) is a classic finding for chronic myelogenous leukemia (CML), but is occasionally seen in acute lymphoblastic leukemia (ALL) and acute myelogenous leukemia (AML).
Toxic granulations in neutrophils (Choice E) are typically seen in overwhelming bacterial infections.
A monoclonal IgM spike is a feature of lymphoplasmacytic lymphoma such as Waldenstrom macroglobulinemia.