Final answer:
Sympathetic blockade from a subarachnoid block typically results in decreased intestinal peristalsis and urinary retention due to the relaxation of smooth muscles in the sympathetic nervous system.
Step-by-step explanation:
The effect of sympathetic blockade from a subarachnoid block can result in various physiological changes in the body due to the inhibition of sympathetic nervous system activity. One of the primary outcomes is the decrease in intestinal peristalsis which is associated with the relaxing effects on smooth muscle in the gastrointestinal tract. This can lead to urinary retention as well, due to decreased bladder muscle tone.
When we consider sympathetic nervous system functions such as pupil dilation, storage of glucose in the liver, increased heart rate, the fight-or-flight response, and the regulation of blood flow through the kidneys, we can determine that sympathetic blockade would oppose these actions. This means we would see a decreased heart rate and, related to the kidneys, a decreased release of renin, which would lead to decreased aldosterone secretion, ultimately affecting fluid retention and blood volume.
Therefore, the correct answer to the student's question is A. decreased peristalsis.