A 60-year-old man presents with fever, fatigue, and anemia. Laboratory findings show leukocytosis with a left shift, elevated C-reactive protein (CRP), and normal platelet count. What is the most likely cause of his leukocytosis? A) Acute viral infection B) Acute bacterial infection C) Chronic myelogenous leukemia D) Hemolytic anemia E) Multiple myeloma Correct Answer: B) Acute bacterial infection Explanation: Leukocytosis with a left shift (increased immature neutrophils) is typical of an acute bacterial infection, as the body releases more neutrophils in response to infection. Acute viral infections (A) may cause leukopenia or a normal leukocyte count, with lymphocytosis rather than neutrophilia. Chronic myelogenous leukemia (C) typically presents with marked leukocytosis and elevated basophils, as well as a characteristic Philadelphia chromosome on cytogenetic testing. Hemolytic anemia (D) can cause anemia and elevated bilirubin, but it is not typically associated with leukocytosis or a left shift. Multiple myeloma (E) presents with elevated monoclonal protein, bone pain, and renal failure rather than leukocytosis. Key Tips for PLAB 1: ✅ Leukocytosis with left shift and fever = Acute bacterial infection ✅ CRP and WBC differential can help confirm infection.
