A 3-year-old presents with bone pain, bowing of legs, widening of wrists, and frontal bossing. X-ray shows widening and fraying of the metaphysis, cupped ends, and Looser's zones. 25-OH vitamin D is 8 ng/mL. His diet is vegetarian, and he has minimal sun exposure. TRAP (tartrate-resistant acid phosphatase) staining on a bone biopsy, if done, would show increased osteoclast activity. What additional electrolyte finding confirms vitamin D-deficient rickets rather than hypophosphatemic rickets?

• A Low serum phosphorus with normal calcium
• B Normal calcium, normal phosphorus, elevated alkaline phosphatase alone
• C High urinary phosphorus with normal serum calcium
• D Low calcium with secondary hyperparathyroidism (elevated PTH) and normal/low phosphorus ✓

Explanation

In vitamin D-deficient rickets, the sequence is: low vitamin D → decreased intestinal Ca absorption → hypocalcemia → secondary hyperparathyroidism (PTH rises) → PTH causes phosphaturia and bone resorption. The classic electrolyte pattern is low/normal calcium, low phosphorus, elevated PTH, low 25-OH-D, and markedly elevated alkaline phosphatase. In X-linked hypophosphatemic rickets (FGF-23 excess), PTH is normal or low-normal and calcium is normal, but phosphorus is low due to primary renal phosphate wasting — a key distinguishing feature.

Reference: Ghai Essential Pediatrics, 10th ed.

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Written and medically reviewed by the StethoPrep medical team.