Step 1: Sympathetic activation and its role. Sympathetic stimulation initiates the "fight or flight" response, redirecting blood to essential organs like muscles and the brain by decreasing renal blood flow.
Step 2: Impact on the glomerulus. Sympathetic stimulation constricts the afferent arteriole, reducing glomerular blood influx and consequently lowering the glomerular filtration rate (GFR). This mechanism aids in water preservation and blood pressure maintenance under stress.
Step 3: Evaluation of other choices. Option (A): Invalid. Afferent arteriole vasodilation would elevate GFR, contrary to sympathetic effects.
Option (B): Valid. Afferent arteriole vasoconstriction, induced by sympathetic stimulation, results in a reduced GFR.
Option (C): Invalid. Sympathetic stimulation substantially influences renal blood flow and GFR.
Option (D): Invalid. Efferent arteriole vasodilation would diminish filtration pressure but is not a characteristic sympathetic response.
The primary outcome of sympathetic stimulation is afferent arteriole vasoconstriction, leading to a GFR reduction.