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?