In the pathogenesis of renal osteodystrophy, what is the initial metabolic derangement driving the cascade toward secondary hyperparathyroidism?

Explanation:
In chronic kidney disease, decreased glomerular filtration leads to phosphate retention (hyperphosphatemia). Elevated phosphate binds to serum calcium, lowering ionized calcium levels. Concurrently, the diseased kidneys fail to convert 25-hydroxyvitamin D to its active form, 1,25-dihydroxyvitamin D, further exacerbating hypocalcemia. This hypocalcemia stimulates the parathyroid glands, resulting in secondary hyperparathyroidism. Reference: Apley & Solomon's System of Orthopaedics and Trauma, 10th Edition.