Which of the following is a physiological reason for leukocytosis?

Leukocytosis is defined as an increased white blood cell (WBC) count, and one physiological reason for this condition is exercise. During physical exertion, the body experiences a temporary increase in white blood cell counts as part of the body's response to stress. This increase occurs due to a variety of factors, including the release of catecholamines (like adrenaline), which mobilizes white blood cells from the spleen and other reservoirs into circulation. This elevation helps enhance the immune response, aiding the body's ability to deal with potential injury or infection that could arise from physical activity.

In contrast, chronic illness is more commonly associated with sustained or abnormal elevations of white blood cells rather than a straightforward physiological response. Low temperature does not normally lead to leukocytosis; in fact, body temperature changes can influence immune function and may lead to different outcomes. Dehydration may also influence blood counts, typically resulting in hemoconcentration rather than a genuine increase in white blood cell production. Thus, exercise is a clear physiological trigger for a temporary increase in WBCs, making it the correct choice.