An 18-month-old girl has rachitic deformities, hypophosphatemia, hypocalcemia, and elevated alkaline phosphatase. Her scalp shows complete alopecia. Her mother's treatment with calcitriol and calcium produced only partial improvement. The most likely defect is:
- A 1-alpha-hydroxylase deficiency in the kidney
- B Mutation of the vitamin D receptor causing end-organ resistance ✓
- C 25-hydroxylase deficiency in the liver
- D Renal phosphate wasting due to FGF23 excess
Correct answer: B. Mutation of the vitamin D receptor causing end-organ resistance
Explanation
Vitamin D-dependent rickets type II results from mutations in the vitamin D receptor, producing resistance to calcitriol with elevated circulating 1,25-(OH)2D levels. Alopecia is the discriminating clinical clue, reflecting failed VDR signaling in hair follicles. Simple 1-alpha-hydroxylase deficiency (type I) lacks alopecia and responds fully to calcitriol. FGF23-mediated phosphate wasting causes isolated hypophosphatemic rickets with normal calcium, unlike this child's hypocalcemia.
Reference: Williams Textbook of Endocrinology, 14th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.