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Regulation of Calcium, Magnesium, and Phosphate

The most important action of calcitriol is to stimulate:

a. calcium deposition in bone.

b. calcium resorption from bone.

c. calcium absorption from the GI tract.

d. calcium reabsorption from the renal tubules.

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

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

a. Adding large amounts of calcium to the diet.

b. The actions of FGF23.

c. The actions of PTH.

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

D. Excess calcium in the GI tract reduces phosphate absorption, while FGF23 and PTH increase renal excretion.

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

a. Bone

b. The GI tract

c. The renal tubules

d. The organelles of tissue cells

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

Magnesium

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

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

c. exists in the body primarily in bone.

d. All of the above statements are true.

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.

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

a. feeding large amounts of phosphate.

b. giving large amounts of saline.

c. injecting PTH.

d. withholding phosphate from the diet.

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