A 18-month-old boy from rural Maharashtra presents with his mother to the pediatric clinic with a 3-month history of progressive irritability, poor weight gain, and delayed motor milestones. On examination, he is noted to have bowing of legs, frontal bossing, and delayed fontanelle closure. Biochemistry shows serum calcium 7.2 mg/dL (normal 8.5–10.5), phosphate 3.8 mg/dL (normal 2.5–4.5), and alkaline phosphatase 180 IU/L (normal 30–120). X-ray of the wrist shows loss of sharp metaphyseal margins and widening of the growth plate. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Vitamin D Deficiency Rickets ### Presentation & Epidemiology **Key Point:** Vitamin D deficiency rickets is the most common form of rickets in children worldwide, particularly in regions with limited sun exposure and inadequate dietary intake of fortified foods. This 18-month-old from rural Maharashtra presents with the classic triad: 1. **Skeletal manifestations:** Bowing of legs, frontal bossing, delayed fontanelle closure 2. **Biochemical findings:** Hypocalcemia (7.2 mg/dL), normal-to-low phosphate, elevated alkaline phosphatase 3. **Developmental delay:** Poor weight gain and delayed motor milestones due to hypocalcemia and metabolic derangement ### Biochemical Pathophysiology ```mermaid flowchart TD A[Vitamin D Deficiency]:::outcome --> B[↓ 1,25-dihydroxyvitamin D]:::outcome B --> C[↓ Intestinal Ca²⁺ absorption]:::outcome C --> D[Hypocalcemia]:::urgent D --> E[↑ PTH secretion]:::outcome E --> F[↑ Bone resorption + Alkaline phosphatase]:::outcome E --> G[↑ Phosphate excretion]:::outcome G --> H[Hypophosphatemia]:::outcome F --> I[Defective mineralization]:::outcome I --> J[Rickets]:::urgent ``` ### Laboratory Differentiation | Feature | Vitamin D Deficiency | Hypophosphatemic | Renal Rickets | |---------|----------------------|------------------|---------------| | **Calcium** | ↓ (low) | Normal | Variable | | **Phosphate** | Normal/Low | ↓↓ (very low) | ↑ (high) | | **Alkaline Phosphatase** | ↑ | ↑↑ | ↑ | | **PTH** | ↑↑ | Normal/↑ | ↑↑ | | **25-OH Vitamin D** | **↓↓ (<20 ng/mL)** | Normal | Normal | | **1,25-(OH)₂ Vitamin D** | ↓ | ↑ | ↓ | | **Etiology** | Dietary/sun exposure | X-linked dominant | CKD, TRF | **High-Yield:** The **serum 25-hydroxyvitamin D level** is the gold standard for diagnosing vitamin D deficiency; levels <20 ng/mL (50 nmol/L) confirm deficiency. ### Radiological Features - Loss of sharp metaphyseal margins (indistinct growth plate) - Widening of growth plate (metaphyseal flare) - Cupping and fraying of epiphysis - Osteopenia with loss of cortical definition - Looser zones (pseudofractures) in severe cases ### Clinical Pearl **Key Point:** In vitamin D deficiency rickets, hypocalcemia is the PRIMARY biochemical abnormality, leading to secondary hyperparathyroidism. This distinguishes it from hypophosphatemic rickets (where phosphate is profoundly low) and renal rickets (where phosphate is elevated due to impaired renal excretion). ### Management 1. **Vitamin D supplementation:** 600,000 IU weekly for 6–8 weeks (loading phase), then maintenance 400–1000 IU daily 2. **Calcium supplementation:** 500–1000 mg elemental calcium daily 3. **Dietary counseling:** Fortified milk, egg yolks, fish liver oil 4. **Sun exposure:** 10–30 minutes daily (depending on latitude and skin type) 5. **Monitor:** Serum calcium, phosphate, alkaline phosphatase, and PTH at 4–6 weeks **Mnemonic:** **RICKETS** = **R**educed 25-OH vitamin D, **I**ncreased PTH, **C**alcium low, **K**etosis absent, **E**levated alkaline phosphatase, **T**ibial bowing, **S**oft bones [cite:Park 26e Ch 9]