A 54-year-old man is brought to the emergency department with 2 days of severe diarrhea and 6 hours of abdominal pain. He is confused and lethargic. His daughter, who is at the bedside, notes that the patient has a history of hypertension and was recently treated for "strep throat." Temperature is 38.7 C (101.6 F), blood pressure is 92/58 mm Hg, pulse is 128/min, and respirations are 24/min. The patient appears agitated. The abdomen is mildly distended, and the patient winces with deep palpation of the left lower quadrant. Laboratory results are as follows: Intravenous fluids and antibiotics are administered. The next morning, the patient's temperature is still elevated. Repeat laboratory examination shows a leukocyte count of 52,000/mm³ with a similar differential. The leukocyte alkaline phosphatase score is high. Which of the following is the likely cause of this patient's hematologic abnormalities?

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11ec25c8_d421_4623_bc81_67abb129358d_MD0003_00 This patient presents with manifestations of acute infectious diarrhea possibly due to Clostridium difficile (recent antibiotic use, severe diarrhea, high fever, left lower quadrant pain). His leukocyte count is dramatically elevated, with a predominance of late neutrophil precursors (metamyelocytes, bands) rather than early precursors (promyelocytes, myelocytes). This, combined with a high leukocyte alkaline phosphatase (LAP) score, suggests leukemoid reaction (LR), characterized by a leukocytosis of >50,000/mm³. Severe infections can cause LR due to the mobilization of mature and immature leukocytes from the bone marrow. Given the severity of the infection, it may take >24 hours for clinical improvement with antibiotics. LR must be differentiated from chronic myelogenous leukemia (CML), a myeloproliferative disorder that also presents with neutrophilia and dramatic leukocytosis. Unlike LR, CML is characterized by a low LAP score (due to cytochemically abnormal neutrophils) and a peripheral smear that features absolute basophilia and early neutrophil precursor cells (Choice B). (Choice A) Chronic lymphocytic leukemia is a lymphoproliferative disorder marked by lymphocytosis, not neutrophilia. (Choice C) Drug reactions commonly present with rash and fever. A severe drug reaction, mediated by mast cells and basophils, can present with anaphylactic symptoms, angioedema, or hypotension. This patient's symptoms of acute, severe diarrhea, abdominal pain, and high fever are more consistent with infection. (Choice E) Myelodysplastic syndrome often presents with infection in the setting of pancytopenia. It may progress to acute leukemia. (Choice F) In secondary polycythemia, the red blood cell count is elevated, typically due to increased erythropoietin (EPO) from hypoxia or an EPO-secreting tumor. This patient's hemoglobin was slightly low. Educational objective: Leukemoid reaction (LR) occurs as a response to severe infection and is marked by leukocyte counts >50,000/mm³. Compared to chronic myeloid leukemia, LR is characterized by the presence of a high alkaline phosphatase score, a greater proportion of late neutrophil precursors (metamyelocytes, bands), and a lack of absolute basophilia.

A 54-Year-Old Man Is Brought to the Emergency Department with 2