A 41-year-old man has recurrent metatarsal stress fractures and premature loss of permanent teeth since his twenties. Radiographs show pseudofractures of the pubic rami. Serum calcium 9.4 mg/dL, phosphate 3.4 mg/dL, PTH normal. The striking finding is a serum alkaline phosphatase of 18 U/L (normal 45 to 125 U/L). What is the diagnosis?
- A Tumour-induced osteomalacia
- B Osteoporosis with insufficiency fractures
- C Hypophosphatasia ✓
- D X-linked hypophosphataemic rickets
Correct answer: C. Hypophosphatasia
Explanation
Hypophosphatasia results from mutations in the ALPL gene encoding tissue-nonspecific alkaline phosphatase. Impaired mineralisation produces pseudofractures resembling osteomalacia, but every other metabolic bone disease raises alkaline phosphatase; a frankly LOW serum ALP is the discriminating clue. Premature tooth loss from absent cementum and elevated plasma pyridoxal-5-phosphate support the diagnosis. Tumour-induced and X-linked hypophosphataemic osteomalacia would show hypophosphataemia and raised ALP.
Reference: Nelson Textbook of Pediatrics, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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