A 18-month-old boy from rural Maharashtra presents with his mother who reports poor feeding, irritability, and recurrent respiratory infections over the past 3 months. On examination, he has bowing of legs, frontal bossing, and delayed fontanelle closure. His wrists and ankles show swelling. Serum calcium is 7.2 mg/dL, phosphate is 3.8 mg/dL, and alkaline phosphatase is elevated at 420 IU/L. X-ray of the wrist shows loss of sharp metaphyseal margins with cupping and fraying. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Vitamin D Deficiency Rickets ### Key Clinical Features **Key Point:** This presentation is classic for nutritional rickets due to vitamin D deficiency — the most common cause of rickets in children worldwide, especially in developing countries with limited sun exposure and dietary vitamin D intake. ### Diagnostic Findings | Feature | Finding | Significance | |---------|---------|-------------| | **Age** | 18 months | Peak incidence of rickets is 6–24 months | | **Skeletal signs** | Bowing of legs, frontal bossing, delayed fontanelle closure | Hallmark of rickets from impaired mineralization | | **Biochemistry** | Low calcium (7.2), low-normal phosphate (3.8), ↑ ALP | Hypocalcemia + secondary hyperparathyroidism | | **Radiological** | Loss of sharp metaphyseal margins, cupping, fraying | Pathognomonic metaphyseal changes of rickets | | **Systemic** | Recurrent infections, poor feeding | Immune dysfunction + poor nutrition in vitamin D deficiency | ### Pathophysiology 1. **Vitamin D deficiency** → impaired intestinal calcium and phosphate absorption 2. **Hypocalcemia** → secondary hyperparathyroidism (PTH ↑) 3. **PTH excess** → increased bone resorption + impaired mineralization 4. **Result:** Defective osteoid mineralization → rickets **High-Yield:** In vitamin D deficiency rickets, serum phosphate is typically low-normal or low (not elevated), and calcium is low. This distinguishes it from hypophosphatemic rickets, where phosphate is markedly low with normal calcium. ### Management 1. Vitamin D supplementation (cholecalciferol 1,00,000 IU weekly × 8 weeks, then maintenance) 2. Calcium supplementation if dietary intake is inadequate 3. Dietary counseling: fortified milk, eggs, fish 4. Sun exposure (15–20 min daily) 5. Monitor biochemistry and radiological healing **Clinical Pearl:** The presence of hypocalcemia with elevated ALP and PTH in a child with skeletal deformities and metaphyseal changes on X-ray is virtually diagnostic of nutritional rickets due to vitamin D deficiency. [cite:Park 26e Ch 9]