An elderly man comes to medical attention because of anemia and multiple infections. Physical examination is remarkable for hepatosplenomegaly. An automated blood count demonstrates pancytopenia. Review of the peripheral smear by a hematologist demonstrates rare, distinctive, neoplastic white cells covered by fine, hairlike projections. Which of the following characteristics would likely be associated with these cells?

The disease is hairy cell leukemia, which, unlike most forms of leukemia, is characterized by pancytopenia rather than increased numbers of circulating cells. This disease tends to affect older men and the characteristic cells (which are not always obviously "hairy") are positive for tarate-resistant acid phosphatase (TRAP). Must know: Associate Birbeck granules with Langerhans cell histiocytosis (histiocytosis X). Associate elevated leukocyte alkaline phosphatase with myeloid metaplasia. Associate Philadelphia chromosome with chronic myeloid leukemia (and occasionally with acute lymphocytic leukemia). Ref: Chandrasoma P., Taylor C.R. (1998). Chapter 19. Neoplasia: III. Biologic & Clinical Effects of Neoplasms. In P. Chandrasoma, C.R. Taylor (Eds), Concise Pathology, 3e