Which of the following is a sign of systemic infection?

An increase in white blood cells (WBC) is indicative of a systemic infection, as the body mounts an immune response to fight off pathogens such as bacteria, viruses, or fungi. White blood cells play a crucial role in the immune system, with certain types, such as neutrophils and lymphocytes, being particularly active during infection. When infection occurs, the body usually responds by producing more white blood cells and releasing them into the bloodstream to help combat the invaders, leading to elevated WBC counts.

In contrast, a decrease in heart rate (which does not correspond with typical responses to infection), a decrease in blood pressure (which can indicate septic shock in a severe infection but isn't a direct sign of infection itself), and stable vital signs would not typically suggest the presence of systemic infection. In the context of a systemic infection, signs often include an increase in heart rate, an increase in WBC, a rise in body temperature, and changes in blood pressure, particularly if the infection progresses to sepsis. Therefore, the increase in WBC is a strong and reliable indicator of systemic infection.