Question

A child presents with rachitic changes in limbs which was not responding to Vitamin D. Investigations done show: Calcium 9.5 mg/dl, Phosphorous 1.6 mg/dl, ALP 814 IU, Sr. PTH 24.2, Sr. Electrolytes, Creatinine and blood gases normal. What is the diagnosis?

• A. Vitamin D dependent rickets type 1
• B. Vitamin D dependent rickets type 2
• C. Hypophosphatemic rickets
• D. Chronic renal failure

Hint:

Focus on which electrolyte is abnormally low, whether calcium is affected, and whether renal function is normal.

Answer 1

The Correct Option is C

Step 1: Analyze the biochemical profile.

• Calcium: 9.5 mg/dl -- normal (normal 8.5-10.5 mg/dl)
• Phosphorus: 1.6 mg/dl -- markedly low (normal 4-6.5 mg/dl in children)
• ALP: 814 IU -- very elevated (active bone disease)
• PTH: 24.2 -- normal-slightly elevated
• Electrolytes, creatinine, blood gases: normal (rules out renal failure and RTA)

Step 2: Classify rickets by the pattern.

• Calcipenic rickets (Vitamin D deficiency, VDDR type 1 & 2): Low calcium + secondary hyperparathyroidism + low phosphorus due to PTH effect. Would respond to Vitamin D unless VDDR type 2 (receptor defect).
• Phosphopenic rickets (Hypophosphatemic rickets): Normal calcium, severely low phosphorus, normal PTH, normal renal function. Primary defect is renal phosphate wasting (increased FGF-23 or PHEX gene mutation). Does NOT respond to Vitamin D alone.
• Chronic renal failure: Elevated creatinine, metabolic acidosis, hyperkalemia -- all normal here, so excluded.

Step 3: Match findings to diagnosis. Normal calcium + very low phosphorus + normal PTH + normal renal function + Vitamin D resistance = Hypophosphatemic rickets (X-linked dominant, due to PHEX gene mutation causing increased FGF-23, leading to renal phosphate wasting).

Conclusion: The diagnosis is Hypophosphatemic rickets.