What condition do the preliminary lab tests indicate?

A 6-year-old girl is brought to a pediatrician by her parents who report that her height is not progressing as they think it should (or like it did for her 8- year-old sister) and her legs look bowed. The patient drinks milk, and other than her shorter stature and bowed legs, she has the normal characteristics of her 6-year-old friends. She takes no medications. Family history is notable for some cousins on the father's side with a similar problem back in the Appalachian Hill country along Virginia/Tennessee border where the family hails from. The pediatrician obtains lab studies that are notable for a calcium level of 7.2 mg/dL (normal, 8.5 to 10.2 mg/dL) with albumin 4.1 g/dL (normal, 3.5 to 4.8 g/dL). Lower extremity radiographs show bowing of the long bones and generalized demineralization. This prompts the measurement of several other laboratory tests, which reveal intact PTH elevated at 866 pg/mL (normal, 11 to 54 pg/mL), 25-hydroxy vitamin D normal at 35 ng/mL (normal, 20 to 57 ng/mL), and 1,25(OH)2D undetectable at less than 1 pg/mL (normal, 20 to 75 pg/mL).

Questions

1. What condition do the preliminary lab tests indicate?

2. What is the significance of 25-hydroxy vitamin D and 1,25(OH)2D levels in the follow-up laboratory tests?

3. Describe the inborn error of metabolism with this patient.

4. What secondary condition will recur if vitamin D treatment is discontinued later in her life?

Solution: