A 18-month-old boy from rural Maharashtra presents with his mother who reports progressive weakness and irritability over 3 months. On examination, he has bowing of legs, frontal bossing, and delayed fontanelle closure. Biochemistry shows serum calcium 7.2 mg/dL, phosphate 3.8 mg/dL, and alkaline phosphatase 180 IU/L (normal <100). X-ray of wrist shows metaphyseal widening and loss of sharp metaphyseal margins. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Vitamin D Deficiency Rickets ### Key Clinical Features **Key Point:** This child presents with the classic triad of nutritional rickets: skeletal deformity (bowing), biochemical evidence of hypocalcemia and elevated alkaline phosphatase, and radiological findings of metaphyseal widening. ### Biochemical Findings | Parameter | Finding | Interpretation | |-----------|---------|----------------| | Serum Calcium | 7.2 mg/dL (low) | Secondary hyperparathyroidism response | | Serum Phosphate | 3.8 mg/dL (low-normal) | Reduced renal reabsorption due to PTH | | Alkaline Phosphatase | 180 IU/L (elevated) | Increased bone turnover | | 25-OH Vitamin D | Typically <20 ng/mL | Confirms vitamin D deficiency | ### Radiological Features of Vitamin D Deficiency Rickets 1. **Metaphyseal changes:** Loss of sharp margins, widening, cupping 2. **Ground glass osteopenia:** Generalized decreased bone density 3. **Looser's zones:** Pseudofractures (pathognomonic but not always present) 4. **Subperiosteal resorption:** Seen at metaphyses ### Pathophysiology 1. Inadequate vitamin D intake or sun exposure → low 25-OH vitamin D 2. Decreased intestinal calcium absorption 3. Secondary hyperparathyroidism develops 4. Increased urinary phosphate → hypophosphatemia 5. Defective mineralization of osteoid → rickets **High-Yield:** In India, vitamin D deficiency rickets is the most common form of rickets in children, especially in populations with limited sun exposure or dietary vitamin D intake. **Clinical Pearl:** The presence of hypocalcemia (not hyperphosphatemia) distinguishes vitamin D deficiency rickets from hypophosphatemic rickets, where phosphate is markedly low but calcium is normal. ### Management - Vitamin D supplementation: 600,000 IU weekly for 6-8 weeks, then maintenance - Calcium supplementation: 500-1000 mg daily - Adequate sun exposure - Dietary counseling [cite:Park 26e Ch 8]