Vitamin D3 (cholecalciferol) undergoes two sequential hydroxylations to form the active hormone 1,25-dihydroxycholecalciferol (calcitriol). The first hydroxylation at C-25 occurs in the liver by CYP2R1/CYP27A1. The second at C-1 occurs in the kidney by CYP27B1 (1-alpha hydroxylase). In a patient with hereditary 1,25-dihydroxyvitamin D resistant rickets (HVDRR, vitamin D dependent rickets type 2), the defect is in the vitamin D receptor (VDR). Which laboratory finding would BEST distinguish HVDRR from CYP27B1 deficiency (VDDR type 1)?

• A Elevated serum 25-hydroxyvitamin D in HVDRR but not VDDR type 1
• B Very high serum 1,25-dihydroxyvitamin D (calcitriol) in HVDRR, whereas it is low in VDDR type 1 ✓
• C Low serum PTH in HVDRR compared to high PTH in VDDR type 1
• D Normal serum calcium in HVDRR versus hypocalcemia in VDDR type 1

Explanation

In hereditary vitamin D resistant rickets (HVDRR, VDDR type 2), the VDR is mutated and cannot respond to calcitriol. The body compensates by upregulating CYP27B1 through PTH and low calcium signals, resulting in markedly elevated serum 1,25-dihydroxyvitamin D (calcitriol) levels. In CYP27B1 deficiency (VDDR type 1), 25-hydroxyvitamin D cannot be converted to 1,25-dihydroxyvitamin D, so calcitriol levels are low or undetectable while 25-OH-D is normal or elevated. Both conditions show hypocalcemia, hypophosphatemia, and secondary hyperparathyroidism. The critical distinguishing test is serum calcitriol: very high in HVDRR, very low in VDDR type 1. Treatment also differs: VDDR type 1 responds to calcitriol supplementation, while HVDRR requires supraphysiologic doses or IV calcium infusions.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

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