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Regulation of Calcium, Magnesium, and Phosphate

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The most important action of calcitriol is to stimulate

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a. calcium deposition in bone.

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b. calcium resorption from bone.

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c. calcium absorption from the GI tract.

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d. calcium reabsorption from the renal tubules.

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(c) Calcitriol has several actions, but the most important one is to ensure adequate supply of calcium from the GI tract.

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Which of the following leads to decreased levels of phosphate in the body?

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a. Adding large amounts of calcium to the diet.

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b. The actions of FGF23.

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c. The actions of PTH.

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d. All of these lead to decreased phosphate in the body.

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(d) Excess calcium in the GI tract reduces phosphate absorption, whereas FGF23 and PTH increase renal excretion.

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In response to a sudden decrease in plasma calcium, what is the source for most of the calcium that restores plasma levels?

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a. Bone

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b. The GI tract

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c. The renal tubules

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d. The organelles of tissue cells

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(a) There is a labile pool of calcium in bone that buffers short-term changes in plasma calcium.

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Magnesium

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a. is reabsorbed from the renal tubules primarily by paracellular diffusion.

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b. has a free cytosolic concentration far higher than that of calcium.

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c. exists in the body primarily in bone.

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d. all of the above statements are true.

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(d) Most magnesium exists in bone (although in a different state than bone calcium). Its tubular reabsorption is paracellular in the proximal tubule and thick ascending limb, and its cytosolic concentration is far higher than that of calcium.

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In a case of acute hypercalcemia, one can rapidly lower plasma calcium and increase urinary calcium excretion by

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a. feeding large amounts of phosphate.

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b. giving large amounts of saline.

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c. injecting PTH.

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d. withholding phosphate from the diet.

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