1. Sepsis
2. Hemorrhage
3. Tissue injury
4. Chronic myeloid leukemia (CML)
5. Cytokine release

Answer 1 is not correct. Answer 4 is correct.

Key Concept: Understand the causes of the leukemoid reaction and distinguish them from malignant causes

The term leukemoid reaction is used to describe a profound leukocytosis (generally defined as a leukocyte count exceeding 25,000 to 30,000/mm³) that is not leukemic in etiology. Leukemoid reactions are the response of normal bone marrow to cytokine release by lymphocytes, macrophages, and other cells in response to infection or trauma. In a leukemoid reaction, the circulating neutrophils are usually mature and are not clonally derived. The major differential diagnosis is with regard to CML. Leukemoid reactions should also be distinguished from leukoerythroblastic reactions: the presence of immature white cells and nucleated red cells in the peripheral blood irrespective of the total leukocyte count. Although less common than leukemoid reactions in adults, leukoerythroblastosis reflects serious marrow stimulation or dysfunction and should prompt bone marrow aspiration and biopsy, unless it occurs in association with severe hemolytic anemia, sepsis in a patient with hyposplenism, or massive trauma. In such patients, trauma, hemorrhage, and infection all will contribute to a potent cytokine release and marrow stimulation. The absence of splenomegaly, leukocyte precursors (myelocytes, metamyelocytes), basophilia, or eosinophilia all point away from CML, and the elevated LAP score confirms the diagnosis of a leukemoid reaction.