What may cause an elevated white blood cell count?

An elevated white blood cell count, or leukocytosis, can indeed be influenced by various conditions, including dehydration. Dehydration can lead to a relative increase in the concentration of white blood cells due to hemoconcentration, which occurs when the volume of plasma decreases while the number of white blood cells remains the same. As the fluid in the bloodstream decreases, the proportions of blood cells become artificially elevated.

The other conditions listed do not cause an elevated white blood cell count in the same manner. While allergies can trigger an increase in certain types of white blood cells, they generally lead to a specific type of response rather than a broad elevation in overall counts. Malnutrition typically results in a decrease in white blood cell levels due to impaired immune function. Extensive hydration would dilute the blood components, potentially leading to a lower relative white blood cell count, not an increase. Thus, dehydration is the most straightforward reason among the choices provided that can lead to an elevated white blood cell count.