单选题 When magnesium sulfate is administered to a client in labor, its action occurs at which of the following sites?
A. Neural-muscular junctions.
B. Distal renal tubules.
C. Central nervous system (CNS).
D. Myocardial fibers.

【正确答案】 A

【答案解析】Because magnesium has chemical properties similar to those of calcium, it will assume the role of calcium at the neural muscular junction. It doesn't act on the distal renal tubules, CNS, or myocardial fibers.