A 36-year-old woman has a cough and fever for 1 week. On physical examination, her temperature is 38.3deg C. She has diffuse crackles in all lung fields. A chest radiograph shows bilateral extensive infiltrates. CBC shows hemoglobin, 13.9 g/ dL; hematocrit, 42%; MCV, 89 mm3; platelet count, 210,000/mm3; and WBC count, 56,000/mm3 with 63% segmented neutrophils, 16% bands, 7% metamyelocytes, 3% myelocytes, 1% blasts, 8% lymphocytes, and 2% monocytes. A bone marrow biopsy is obtained and shows normal maturation of myeloid cells. Which of the following is the most likely diagnosis?

Marked leukocytosis and immature myeloid cells in the peripheral blood can represent an exaggerated response to infection (leukemoid reaction), or it can be a manifestation of chronic myelogenous leukemia (CML). Normal maturation of myeloid cells in the marrow rules out CML. Although not provided in this case, a leukocyte alkaline phosphatase (LAP) score is high in the more differentiated cell population of reactive leukocytosis, whereas in CML, the LAP score is low. The Philadelphia chromosome (present in most CML cases) is lacking in patients with leukemoid reactions. Hairy cell leukemia is accompanied by peripheral blood leukocytes that mark with tartrate-resistant acid phosphatase. Hodgkin lymphoma is not characterized by an increased WBC count. A myelodysplastic syndrome is a stem cell maturation disorder involving all nonlymphoid cell lineages, not just granulocytes.