PTH increases calcium absorption by which mechanism?

• A. Directly increases intestinal calcium absorption
• B. Increasing gastric acid secretion to enhance calcium ionization
• C. Inhibiting intestinal calcium channels
• D. Stimulating the kidney to produce active vitamin D

Answer: (D)

PTH increases calcium absorption indirectly by stimulating the kidney to produce the active form of vitamin D. It enhances 1α-hydroxylase activity in the proximal tubules, converting 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D (calcitriol). Calcitriol then acts on intestinal cells to upregulate calcium transport proteins and channels, boosting transcellular calcium uptake from the gut into the bloodstream. So the mechanism is that PTH raises active vitamin D production in the kidney, which in turn increases intestinal calcium absorption. The other options don’t fit because PTH does not directly boost gut absorption, does not work by increasing gastric acid secretion to ionize calcium, and does not inhibit intestinal calcium channels.