The main effect of 1,25-dihydroxyvitamin D is to
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______.

Question

The main effect of 1,25-dihydroxyvitamin D is to 
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______.

BlackTom AI · Accepted Answer

The question presents a basic physiology concept regarding the main action of 1,25-dihydroxyvitamin D. 
Option 1: 'dissolve bony matrix, releasing calcium into the blood' This describes bone resorption, which is primarily mediated by parathyroid hormone (PTH) and osteoclast activity, not the direct action of actively circulating vitamin D. While vitamin D supports bone mineralization, this option overstates a resorptive action that is not the main effect of 1,25-dihydroxyvitamin D.

Option 2: 'increase absorption of dietary calcium in the intestines' This matches the well-established role of 1,25-dihydroxyvitamin D, also known as calcitriol, which enhances intestinal calcium absorption by upregulating calcium-binding proteins and transporters in enterocytes. This effect is central to vitamin D biology and helps maintain calcium homeostasis.

Option 3: 'increase calcium conservation in the kidneys' The kidneys do reabsorb calcium, and vitamin D does influence renal calcium handling, but the principal effect of 1,25-dihydroxyvitamin D is not primarily to conserve calcium in the kidneys. Its major intestinal action is more emphasized in standard teaching.

Option 4: 'increase calcium excretion in the kidneys' This is contrary to the known function; calcitriol does not increase renal calcium excretion. In fact, it tends to promote calcium reabsorption rather than excretion, aligning with calcium conservation rather than loss.

Option 5: 'darken the skin, to protect it from solar radiation' This option is unrelated to the biochemical actions of 1,25-dihydroxyvitamin D. Skin darkening is related to melanin production and UV exposure, not to the vitamin D metabolic pathway or its principal action.

In summary, the option that best reflects the established main effect of 1,25-dihydroxyvitamin D is the one describing enhanced intestinal calcium absorption, as this directly supports calcium homeostasis through increased uptake from the diet. The other choices either describe processes governed by different hormones, misrepresent vitamin D’s renal effects, or are irrelevant to its physiological role.

The main effect of 1,25-dihydroxyvitamin D is to Blank ______.单项选择题

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