What is the most likely explanation for a leukemoid reaction in a previously healthy elderly man presenting with fever and cough?

A leukemoid reaction is characterized by an extremely high white blood cell count and is often a response to severe infections or other stressors. In the case of a previously healthy elderly man presenting with fever and cough, the most reasonable explanation for the leukemoid reaction is that it is due to a probable infection.

Infections can stimulate the bone marrow to produce more white blood cells as the body attempts to fight off the invading pathogens. This is particularly important in an elderly patient, where an infection can lead to a more robust immune response as the body may be compensating for reduced function in other areas. The acute nature of the symptoms (fever and cough) supports the likelihood of an underlying infection such as pneumonia or another respiratory tract infection, which are common causes of leukemoid reactions in older adults.

Other conditions listed, such as essential thrombocythemia and acute leukemia, typically involve chronic changes in blood cell production or quality, rather than an acute reaction to infection. Cytomegalovirus (CMV) infection can cause elevated white blood cell counts but is less likely to present predominantly as a leukemoid reaction compared to typical bacterial or viral infections. Therefore, the presence of fever and cough strongly indicates that the leukemoid reaction is