Ans. is 'a' i.e. PTH Guyton states that whenever there is prolonged calcium excess or deficiency only the PTH hormone is really important in maintaining a normal serum calcium ion concentration. When there is prolonged deficiency of calcium in the diet, increased secretion of PTH can stimulate enough calcium absorption from the bones to maintain the normal serum calcium ion concentration for 1 year or more (But eventually even the bones will run out of calcium).Calcium ion homeostasis 3 hormones are primarily involved in the regulation of calcium metabolism and maintaining a constant serum calcium ion concentration.1, 25-Dihdydroxycholecalciferolis a steroid hormone formed from vitamin D by successive hydroxylation in the liver and kidneys.its primary action is to increase calcium and phosphate absorption from the intestineParathyroid hormone (PTH)secreted by parathyroid glandmain action is to mobilize calcium from bones and increase urinary phosphate excretion.increasing the formation in the kidney of 1,25 dihydroxycholecalciferols from vitamin D.* Calcitoninsecreted by parafollicular or C cells in thyroidit decreases plasma calcium ion concentration by inhibiting bone resorptionin adults calcitonin plays only minor role in calcium metabolism.Effects of these 3 hormones on calcium & phosphate ion concentration Calcium ionPhosphate ion1,25 Dihydroxycholecalciferol||PTH||Calcitonin| Action of Vitamin Dpromotes intestinal absorption of calcium and phosphateincreases calcium and phosphate ion absorption in renal tubules, thus decreasing their excretion in urine.Actions of PTHincreases calcium and phosphate absorption from the boneincreases renal tubular reabsorption of calcium ionsdecreases tubular reabsorption of phosphate ion thus increases phosphate excretion by the kidneys.increases both calcium and phosphate absorption from the intestines by increasing the formation in the kidneys of 1,25 dihydroxycholicalciferol from vitamin D.