A 38-year-old man has progressive bone pain and a waddling gait. Labs: calcium 9.1 mg/dL, phosphate 1.6 mg/dL, alkaline phosphatase 320 U/L, PTH mildly elevated, 25-hydroxyvitamin D 34 ng/mL, 24-hour urinary phosphate high with normal creatinine clearance. MRI shows a small soft tissue mass in the left thigh. The underlying abnormality is:
- A Tumor secretion of FGF23 causing renal phosphate wasting ✓
- B Tumor secretion of PTHrP mimicking hyperparathyroidism
- C Autoimmune activation of the calcium sensing receptor
- D Tumor production of sclerostin suppressing osteoblast function
Correct answer: A. Tumor secretion of FGF23 causing renal phosphate wasting
Explanation
This is tumor induced osteomalacia, usually from a benign phosphaturic mesenchymal tumor. The tumor secretes FGF23, which suppresses renal tubular sodium phosphate cotransporters and downregulates 1-alpha hydroxylase, producing renal phosphate wasting, low or inappropriately normal 1,25 dihydroxyvitamin D, and osteomalacia despite adequate 25-hydroxyvitamin D. PTHrP causes hypercalcemia of malignancy, not hypophosphatemia, which eliminates option B. Treatment is surgical removal of the tumor plus phosphate and calcitriol replacement.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.