In CKD-MBD, how does CKD affect vitamin D and bone turnover?

• A. CKD increases vitamin D activation
• B. CKD reduces PTH release
• C. CKD impairs vitamin D activation, increasing bone turnover and vascular calcification risk
• D. CKD does not affect bone turnover

Answer: (C)

In CKD-MBD, the kidney’s ability to activate vitamin D is reduced because loss of functioning nephrons lowers 1α-hydroxylase activity. That means less active vitamin D (calcitriol) is made, which decreases intestinal calcium absorption. The resulting hypocalcemia stimulates secondary hyperparathyroidism, and high PTH drives increased bone turnover as bone is resorbed and remodeled more than normal. The altered mineral balance (high phosphate from poor excretion and reduced calcitriol) also promotes vascular calcification risk. So CKD impairs vitamin D activation, leading to higher bone turnover and greater risk of vascular calcification.