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Pseudohypoparathyroidism
50%
17/34
Renal osteodystrophy
15%
5/34
Vitamin D deficiency rickets
6%
2/34
Vitamin D resistant rickets
24%
8/34
Type II vitamin D dependent rickets
3%
1/34
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This patient is presenting with bilateral bowing of the tibia and femur suggesting inadequate bone mineralization due to metabolic disease. His lab values (normal vitamin D, low phosphorus, elevated alkaline phosphatase, and normal PTH) are consistent with vitamin D resistant rickets (also known as familial hypophosphatemic rickets). Familial hypophosphatemic rickets is the most common form of rickets. It is caused by a X-linked dominant mutation in the PHEX gene leading to an inability of the renal tubules to absorb phosphate (glomerular filtration rate remains normal). This leads to inadequate mineralization of bone and commonly presents as bilateral tibial bowing between the ages of 6 months and 2 years. Laboratory changes include a low serum phosphorous and an elevated alkaline phosphatase. Serum calcium is usually normal (or the low end of normal). High dose vitamin D3 and corrective surgery is currently the standard of care for hypophosphatemic rickets. Figure A is the physical exam finding demonstrating bilateral tibial bowing seen in rickets. Incorrect Answers: Answer 1: Pseudohypoparathryoidism is caused by a defect in the parathyroid receptor making it unresponsive to PTH. These patients will have decreased serum calcium and vitamin D and elevated phosphorous and PTH. Answer 2: Renal osteodystrophy is characterized by the inability to excrete phosphorous as a result of renal failure. As a result of chronic hyperphosphatemia, patients develop hypocalcemia and have increased levels of alkaline phosphatase and PTH. Answer 3: Patients with vitamin D deficient rickets have a nutritional lack of vitamin D and will have decreased serum calcium, phosphorous, and vitamin D in the setting of elevated alkaline phosphatase and PTH. Answer 5: Patients with type II vitamin D dependent rickets have an autosomal recessive mutation leading to a defect in the intracellular receptor for 1,25-dihydroxyvitamin D. They will present with a decreased serum calcium and phosphorous and have an elevated alkaline phosphatase and high levels of vitamin D. Bullet Summary: Vitamin D resistant rickets presents with soft bones (bowing of the lower extremities) and laboratory findings demonstrating normal vitamin D, low phosphorus, elevated alkaline phosphatase, and a normal PTH.
3.8
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