Urology>>>>>Renal Pathophysiology
Question 3#

Which of the following is TRUE regarding bone mineralisation?

A. The actions of Vitamin D are exerted largely through the liver
B. Vitamin D3 requires two hydroxylations, first in liver and second in the kidney
C. During periods of hypercalcemia, PTH synthesis and secretion are increased while degradation is decreased
D. The renal effects of PTH are to increase active calcium reabsorption at the level of the proximal tubule and decrease phosphate reabsorption mainly in the distal tubule
E. PTH stimulates calcitriol production by decreasing 1α-hydroxylase levels

Correct Answer is B


Answer B

Normal regulation of bone mineralisation occurs through maintenance of serum calcium and phosphorus levels and is achieved through the actions of vitamin D and parathyroid hormone (PTH). The actions of both hormones are exerted largely through the kidney. Vitamin D3, which has minimal biological activity, requires two hydroxylations – first occurs in the liver and the second within the tubular cell, through the action of 25-hydroxylase to form 25-hydroxycholecalciferol (calcidiol). The calcidiol is then transported to the kidney, where it is filtered and reabsorbed by renal tubular cells. These cells contain both 1α-hydroxylase and 24α-hydroxylase, and produce either the active 1,25-dihydroxycholecalciferol (calcitriol) or the inactive 24,25-dihydroxycholecalciferol. Parathormone increases the active calcium reabsorption at the level of the distal tubule. Secondly, it decreases phosphate reabsorption in the proximal convoluted tubule (and the distal tubule, to a lesser degree) and thirdly, it stimulates calcitriol production by increasing 1α-hydoxylase levels and decreasing 24α-hydroxylase levels.