Activation of sympathetic nerves that innervate the kidney leads to a decreased urine production. knowing that fact, what do you think the sympathetic nerves do to the afferent arteriole

I think the answer is constriction. 
The kidney is richly innervated organ and has both efferent and afferent nerves. Renal sympathetic nerve activity can modulate central sympathetic outflow and therefore, sympathetic nerve activity at the whole body level. Their activation in the kidney leads to decreased urine production and also casue constriction to the afferent arteriole.

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