An 8-year-old boy presents with bowing of the lower limbs, premature exfoliation of deciduous teeth, and a history of recurrent fractures. Radiographs show features resembling rickets. Serum calcium is 9.6 mg/dL, phosphate is 3.8 mg/dL, vitamin D is adequate, and serum alkaline phosphatase is markedly LOW. The most likely diagnosis is:

• A X-linked hypophosphatemic rickets
• B Fibrous dysplasia (McCune-Albright)
• C Vitamin D dependent rickets type I
• D Hypophosphatasia ✓

Explanation

Hypophosphatasia results from deficient tissue nonspecific alkaline phosphatase activity, impairing skeletal mineralization while calcium, phosphate, and vitamin B remain normal. Low alkaline phosphatase with rickets-like radiographs and premature tooth loss is pathognomonic. X-linked hypophosphatemic rickets shows renal phosphate wasting with LOW phosphate and normal or high alkaline phosphatase, which directly contradicts this child's normal phosphate and low enzyme level, killing option A. Enzyme replacement with asfotase alfa is available.

Reference: Nelson Textbook of Pediatrics, 22nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.