What would likely increase the viscosity of blood?

Elevated hematocrit levels indicate a higher concentration of red blood cells in the blood, which increases the blood's viscosity. Viscosity refers to the thickness or resistance to flow of a fluid, and in the case of blood, it is affected significantly by the cellular components present. As the proportion of red blood cells in relation to plasma rises, the blood becomes thicker and less able to flow easily through blood vessels, leading to increased viscosity.

The other options, while having their own implications within the context of blood flow and viscosity, do not result in an increase. For instance, low hematocrit levels suggest fewer red blood cells and would therefore decrease viscosity. High fluid temperature would generally decrease viscosity, as warmer fluids flow more easily. Increased blood flow rate can affect shear stress but does not directly contribute to increased viscosity based on hematocrit levels; rather, it can enhance flow dynamics in certain contexts. Thus, elevated hematocrit levels is the most direct and significant factor leading to increased blood viscosity.