## Vitamin D-Dependent Rickets Type 1 (VDDR-1) ### Pathophysiology VDDR-1 is an autosomal recessive disorder characterized by a deficiency of **1α-hydroxylase**, the enzyme responsible for converting 25-hydroxyvitamin D [25(OH)D] to the active metabolite **1,25-dihydroxyvitamin D [1,25(OH)₂D]** in the proximal tubule of the kidney. ### Biochemical Profile | Parameter | Finding | |-----------|----------| | 25(OH)D | Normal or elevated | | 1,25(OH)₂D | **Low** | | Parathyroid hormone (PTH) | Elevated (secondary hyperparathyroidism) | | Serum calcium | Low | | Serum phosphate | Low | | Alkaline phosphatase | Elevated | ### Clinical Features - Onset: typically 6 months to 2 years of age - Severe rickets with hypocalcemia and secondary hyperparathyroidism - Responsive to **high-dose calcitriol** (active vitamin D) therapy - Does NOT respond to standard vitamin D supplementation alone **Key Point:** The distinguishing feature of VDDR-1 is the **low 1,25(OH)₂D level despite normal or elevated 25(OH)D**, indicating a defect in the final activation step of vitamin D metabolism. **High-Yield:** VDDR-1 is treatable with calcitriol (0.5–2 μg/day), bypassing the enzymatic defect. ### Contrast with VDDR-2 VDDR-2 involves **end-organ resistance** to calcitriol, resulting in extremely high 1,25(OH)₂D levels despite clinical rickets — a fundamentally different biochemical pattern. 
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