Question:medium
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 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?
Show Hint
Focus on which electrolyte is abnormally low, whether calcium is affected, and whether renal function is normal.
Updated On: Jun 23, 2026
- Vitamin D dependent rickets type 1
- Vitamin D dependent rickets type 2
- Hypophosphatemic rickets
- Chronic renal failure
Show Solution
The Correct Option is C
Solution and Explanation
Biochemical clue analysis:
Ca = 9.5 mg/dl (normal), PO4 = 1.6 mg/dl (very low), ALP = 814 IU (elevated), PTH = 24.2 (near normal), Creatinine and blood gases = normal.
Differential diagnosis of Vitamin D resistant rickets:
Key mechanism: Hypophosphatemic rickets is caused by increased FGF-23 (due to PHEX gene mutation in X-linked form), which inhibits renal tubular phosphate reabsorption (Na-Pi cotransporter) and suppresses 1-alpha-hydroxylase. The primary defect is phosphopenic, not calcipenic.
\[\boxed{\text{Hypophosphatemic rickets}}\]
Ca = 9.5 mg/dl (normal), PO4 = 1.6 mg/dl (very low), ALP = 814 IU (elevated), PTH = 24.2 (near normal), Creatinine and blood gases = normal.
Differential diagnosis of Vitamin D resistant rickets:
| Type | Ca | PO4 | PTH | Response to Vit D |
| VDDR Type 1 | Low | Low | High | Yes (pharmacological dose) |
| VDDR Type 2 | Low | Low | High | No |
| Hypophosphatemic rickets | Normal | Very low | Normal | No (needs phosphate + calcitriol) |
| Renal osteodystrophy | Low | High | High | Partial |
Key mechanism: Hypophosphatemic rickets is caused by increased FGF-23 (due to PHEX gene mutation in X-linked form), which inhibits renal tubular phosphate reabsorption (Na-Pi cotransporter) and suppresses 1-alpha-hydroxylase. The primary defect is phosphopenic, not calcipenic.
\[\boxed{\text{Hypophosphatemic rickets}}\]
Was this answer helpful?
0
