What causes CKD-related secondary hyperparathyroidism?

• A. Increased phosphate excretion
• B. Impaired phosphate excretion and reduced active vitamin D leading to hypocalcemia, stimulating PTH secretion
• C. Increased calcium absorption raising calcium levels
• D. Decreased PTH production

Answer: (B)

In CKD, phosphate handling and vitamin D activation are disrupted, leading to low calcium and a compensatory rise in PTH. The kidneys lose the ability to excrete phosphate efficiently, causing phosphate to accumulate. High phosphate lowers the level of free calcium by binding it and promoting withdrawal from the bloodstream. At the same time, the diseased kidneys produce less 1,25-dihydroxyvitamin D (calcitriol), which reduces intestinal calcium absorption. The combination of hyperphosphatemia, hypocalcemia, and reduced calcitriol drives the parathyroid glands to secrete more PTH to try to raise calcium levels. Over time this results in CKD-related secondary hyperparathyroidism.

The other options don’t fit: increasing phosphate excretion would reduce phosphate buildup, which doesn’t occur in CKD; higher calcium absorption would raise calcium and suppress PTH; and decreased PTH production would not explain the marked rise in PTH seen in this condition.