A 3-month-old infant has suffered recurrent episodes of fever, skin pustules, and pneumonia since the first week of life. Repeated blood counts have shown absolute neutrophil count (ANC) to be less than 500/mm3, whereas hemoglobin and platelet count are normal. The most likely diagnosis is

The patient has a severe congenital cytopenia that is limited to the neutrophil lineage; thus, aplastic anemia and acute leukemia, which affect all hemic lineages, are very unlikely. Cyclic neutropenia is characterized by a periodicity in absolute neutrophil count (ANC) rather than a constant lowering of ANC. The use of granulocyte colony-stimulating factor to stimulate neutrophil production has markedly improved the prognosis for patients with both Kostmann agranulocytosis and cyclic neutropenia.