What could cause lymphocytosis?

Lymphocytosis, which is characterized by an increased number of lymphocytes in the blood, is commonly associated with viral infections. This is due to the immune system's response to combat the viral pathogens, leading to an increase in lymphocyte production as these cells are crucial for the adaptive immune response. During a viral infection, the body recognizes foreign viral particles, and lymphocytes proliferate to mount an effective immune response, thus resulting in lymphocytosis.

The other choices do not directly cause lymphocytosis. Dehydration can lead to hemoconcentration, but it does not specifically cause an increase in lymphocyte count. Corticosteroids are known to cause lymphopenia, a decrease in lymphocyte levels, rather than an increase. Low sodium levels are typically associated with other electrolyte imbalances and are not directly related to changes in lymphocyte counts. Thus, the most appropriate and well-established cause of lymphocytosis in this context is a viral infection.