During stress, the sympathetic nervous system's activation leads to which of the following effects?

The activation of the sympathetic nervous system during stress triggers the "fight or flight" response, which prepares the body for rapid physical action. One of the primary effects of this activation is increased cardiac output. This occurs through several mechanisms: the heart rate increases, and the strength of each contraction of the heart is enhanced, allowing for more blood to be pumped throughout the body. This heightened cardiac output is crucial during stress or emergencies, as it ensures that vital organs and muscles receive enough oxygen and nutrients to function optimally.

In contrast, other options presented do not align with the typical physiological responses associated with sympathetic activation during stress. For instance, salivation generally decreases as the body prioritizes immediate survival responses over digestion. Additionally, pupil dilation results from sympathetic activation, not constriction, allowing more light into the eyes for better vision in critical situations. Lastly, heart rate variability often decreases under stress due to increased sympathetic tone and reduced parasympathetic influence, indicating a less adaptable heart rate in response to varying demands. Hence, increased cardiac output stands out as the correct choice in this context.