Which physiological change can explain an increase in hematocrit levels?

An increase in hematocrit levels can be attributed to dehydration because dehydration leads to a reduction in plasma volume, which is the liquid component of blood. When fluid is lost from the body, whether through sweating, vomiting, or other methods, the concentration of red blood cells in the remaining blood volume rises. This concentration is reflected in a higher hematocrit value, which is a measurement of the proportion of blood volume that is occupied by red blood cells.

In contrast, excessive hydration dilutes the blood, potentially lowering hematocrit levels. Increased blood volume from fluid retention or infusion would not lead to an increase in hematocrit, as it would similarly dilute the concentration of red blood cells. Furthermore, a decreased red blood cell count would directly lower hematocrit levels, since the hematocrit is specifically a measure of the percentage of blood composed of red blood cells. Therefore, dehydration is the physiological change that leads to an increase in hematocrit levels.