A 15-month-old child presents with 'rachitic rosary', widened wrists, delayed dentition, and muscle hypotonia. X-ray shows cupped, frayed metaphyses and widened growth plates. Serum findings: calcium 7.5 mg/dL, phosphorus 2.8 mg/dL, ALP 900 U/L, 25-OH vitamin D 8 ng/mL. Which statement best differentiates nutritional rickets from X-linked hypophosphatemic rickets (XLH)?
- A In nutritional rickets, hypocalcemia and secondary hyperparathyroidism drive phosphaturia ✓
- B In XLH, serum 1,25-(OH)2D3 levels are appropriately elevated in response to hypophosphatemia
- C XLH responds to high-dose vitamin D supplementation alone
- D Hypocalcemic tetany is common in XLH but not in nutritional rickets
Explanation
In nutritional rickets, the primary defect is vitamin D deficiency → hypocalcemia → secondary hyperparathyroidism → PTH-mediated phosphaturia (FGF-23 also increases) → hypophosphatemia. Thus hypocalcemia and elevated PTH drive the phosphaturia. In XLH (PHEX gene mutation), FGF-23 is constitutively elevated, causing isolated renal phosphate wasting; serum calcium is normal, PTH is usually normal, and 1,25-(OH)2D levels are inappropriately normal or low (FGF-23 suppresses 1-alpha-hydroxylase). This distinguishes it from nutritional rickets. XLH does not respond to vitamin D alone and requires phosphate supplementation plus active vitamin D (calcitriol) or burosumab (anti-FGF-23 antibody).
Reference: Ghai Essential Pediatrics, 10th ed.
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