A 5-year-old boy from northern India presents with bowing of legs, frontal bossing, rachitic rosary, and delayed tooth eruption. Serum calcium is 7.8 mg/dL, phosphate 3.2 mg/dL, alkaline phosphatase 280 IU/L, and PTH 210 pg/mL. Which investigation is most specific for confirming nutritional rickets and assessing severity?

Explanation

## Investigation of Choice for Nutritional Rickets **Key Point:** Serum 25-hydroxyvitamin D is the most specific biochemical test to confirm nutritional rickets and quantify the degree of vitamin D deficiency. It directly reflects the etiology and guides treatment intensity. ### Why 25(OH)D is the Best Choice | Investigation | Purpose | Specificity | Utility in Rickets | |---|---|---|---| | 25(OH)D level | Confirms vitamin D deficiency | **High** | Diagnostic + prognostic | | Wrist X-ray | Shows structural changes | Low (non-specific) | Assesses skeletal deformity | | FGF23 level | Evaluates phosphate metabolism | Moderate | Useful in hypophosphatemic rickets | | Urinary phosphate | Assesses renal handling | Low | Non-specific marker | **High-Yield:** In nutritional rickets: - **25(OH)D <10 ng/mL** = Severe deficiency - **25(OH)D 10–20 ng/mL** = Moderate deficiency - **25(OH)D 20–30 ng/mL** = Mild deficiency This grading correlates with clinical severity and guides treatment dosing. ### Clinical Correlation **Clinical Pearl:** The child's clinical features (bowing, frontal bossing, rachitic rosary, delayed dentition) and biochemistry (hypocalcemia, elevated PTH, high ALP) are classic for nutritional rickets. A 25(OH)D level will typically be <10 ng/mL in this presentation, confirming vitamin D deficiency as the primary cause and allowing risk stratification for treatment. ### Why Other Tests Are Inferior **Wrist X-ray:** Shows metaphyseal widening, loss of sharp metaphyseal margins, and subperiosteal resorption—findings that are **structural** but not **etiological**. X-rays cannot distinguish nutritional rickets from other forms (hypophosphatemic, hypocalcemic) or assess biochemical severity. **FGF23:** Elevated in hypophosphatemic rickets (X-linked, autosomal dominant/recessive). In nutritional rickets, FGF23 is typically normal or low. It is not the first-line test for nutritional rickets. **Urinary phosphate:** Reflects renal phosphate handling but is non-specific and not diagnostic of rickets etiology. **Mnemonic:** **25-SPECIFIC** — 25(OH)D is the most SPECIFIC test for vitamin D-deficient rickets.