Passage IV
In human blood, calcium (Ca2+) concentration is regulated by vitamin D3. First, vitamin D3 is converted to calcidiol (CD) in the liver. An elevated concentration of CD inhibits further conversion of vitamin D3. Next, CD is converted to calcitriol (CT) in the kidneys in a reaction that requires parathyroid hormone (PTH). An elevated concentration of CT increases Ca2+ concentration, which in turn inhibits further release of PTH. See Figure 1.
Figures 2 and 3, respectively, show how dietary consumption of vitamin D3 affects relative blood CD concentration and how blood Ca2+ concentration affects relative blood CT concentration.
Figures adapted from Hall, John E., Guyton and Hall Textbook of Medical Physiology, 12th ed. ©2011 by Saunders Elsevier.
26. According to the given information, the inhibition of the conversion of vitamin D3 to CD is caused by:
Your Answer is
Correct Answer is J
Explanation
Looking at Figure 1, in the process from Vitamin D3 to CD, the dotted arrow reflects the relationship of inhibition, so it is CD itself that hinders the conversion of Vitamin D3 into CD. It should be that the higher the concentration of CD, the stronger the inhibitory relationship . So option J is reasonable.