A 4-year-old boy has short stature and bowing of legs. Calcium is 9.8 mg/dL, phosphate is 1.8 mg/dL, alkaline phosphatase elevated, 25(OH)D level is 35 ng/mL, and urinary phosphate excretion is high. His maternal uncle has similar deformities. What is the most likely diagnosis?
- A X-linked hypophosphatemic rickets ✓
- B Nutritional vitamin D deficiency rickets
- C Vitamin D dependent rickets type I
- D Distal renal tubular acidosis
Correct answer: A. X-linked hypophosphatemic rickets
Explanation
X-linked hypophosphatemic rickets results from PHEX mutation causing renal phosphate wasting via excess FGF23. The hallmark is hypophosphatemia with NORMAL serum calcium, normal or low-normal 25(OH)D, elevated ALP, and phosphaturia, inherited as an X-linked dominant trait affecting males and females across generations. Nutritional rickets would show low calcium, low 25(OH)D, and secondary hyperparathyroidism.
Reference: Nelson Textbook of Pediatrics, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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