A 62-year-old man with CKD stage 4 (eGFR 22 mL/min) presents with serum phosphate 6.8 mg/dL, calcium 8.1 mg/dL, PTH 380 pg/mL, and FGF-23 markedly elevated. This pattern describes CKD-Mineral Bone Disorder (CKD-MBD). The sequence of events in CKD-MBD initiation is best described as:
- A Hyperphosphatemia → elevated PTH → reduced 1,25-OH vitamin D → hypocalcemia
- B Hypocalcemia is the first abnormality, triggering PTH secretion and subsequent phosphate retention
- C Reduced 1,25(OH)2 vitamin D and elevated FGF-23 occur EARLY before overt hyperphosphatemia ✓
- D Reduced GFR → decreased phosphate excretion → hyperphosphatemia → hypocalcemia → PTH rise
Explanation
In CKD-MBD, FGF-23 (fibroblast growth factor-23) elevation is the earliest detectable abnormality, occurring when GFR is still 50–70 mL/min. FGF-23 is secreted by osteocytes in response to phosphate loading and inhibits 1α-hydroxylase, reducing 1,25(OH)2D3 synthesis, and promotes phosphaturia (protective initially). Reduced 1,25-dihydroxyvitamin D causes intestinal calcium malabsorption and reduces calcium-sensing receptor expression in parathyroid, stimulating PTH. Overt hyperphosphatemia develops later as GFR falls below ~30 mL/min when tubular phosphate excretion becomes inadequate. FGF-23 and vitamin D deficiency thus precede hyperphosphatemia and hypocalcemia in the CKD-MBD timeline.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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