Answer to Question 1
To make vitamin D, ultraviolet rays from the sun hit a precursor in the skin and convert it to previtamin D3, which is converted to vitamin D3 with the help of the body's heat. To activate vitamin Dwhether made in the body or consumed from the diettwo hydroxylation reactions must occur. First, the liver adds an OH group, and then the kidneys add another OH group to produce the active vitamin. As you might expect, diseases affecting either the liver or the kidneys can interfere with the activation of vitamin D and produce symptoms of deficiency.
Answer to Question 2
Night blindness is one of the first detectable signs of vitamin A deficiency and permits early diagnosis. In night blindness, the person loses the ability to recover promptly from the temporary blinding that follows a flash of bright light at night or to see after dark. In many parts of the world, after the sun goes down, vitamin Adeficient people become night-blind. They often cling to others or sit still, afraid that they may trip and fall or lose their way if they try to walk alone.
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