Question:medium
A 2 year old boy has Vitamin D refractory Rickets. Investigations show serum calcium 9 mg/dl, phosphate 2.4 mg/dl, alkaline phosphatase 1040 IU. Parathyroid hormone level and bicarbonate levels are normal. The most probable diagnosis is
A 2 year old boy has Vitamin D refractory Rickets. Investigations show serum calcium 9 mg/dl, phosphate 2.4 mg/dl, alkaline phosphatase 1040 IU. Parathyroid hormone level and bicarbonate levels are normal. The most probable diagnosis is
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Normal bicarbonate rules out RTA; normal calcium and PTH rule out vitamin D dependent rickets; low phosphate with normal calcium points to hypophosphatemic rickets.
Updated On: Jul 8, 2026
- Distal Renal Tubular Acidosis
- Hypophosphatemic Rickets
- Vitamin D Dependent Rickets
- Proximal Renal Tubular Acidosis
Show Solution
The Correct Option is B
Solution and Explanation
Step 1: List what each rachitic cause should do to the labs.
Distal RTA: low bicarbonate (acidosis), variable calcium and phosphate.
Proximal RTA: low bicarbonate (acidosis), often with glucosuria and other Fanconi features.
Vitamin D dependent rickets: low calcium, high PTH (secondary hyperparathyroidism), variable phosphate.
Hypophosphatemic rickets: normal calcium, low phosphate, normal PTH, normal bicarbonate.
Step 2: Compare this list against the given child.
The child has calcium $9$ mg/dl (normal), phosphate $2.4$ mg/dl (low), alkaline phosphatase $1040$ IU (high), PTH normal, bicarbonate normal.
Step 3: Match line by line.
Bicarbonate being normal removes both RTA types, since acidosis is their hallmark. Calcium being normal, not low, and PTH being normal, not raised, removes vitamin D dependent rickets, since that condition runs on hypocalcemia driving up PTH.
Step 4: Confirm the fit.
Only the hypophosphatemic rickets row matches every value: normal calcium, low phosphate, normal PTH, normal bicarbonate, with a raised alkaline phosphatase from active bone disease.
Step 5: State the answer.
The most probable diagnosis is Hypophosphatemic Rickets.
\[ \boxed{\text{Hypophosphatemic Rickets}} \]
Distal RTA: low bicarbonate (acidosis), variable calcium and phosphate.
Proximal RTA: low bicarbonate (acidosis), often with glucosuria and other Fanconi features.
Vitamin D dependent rickets: low calcium, high PTH (secondary hyperparathyroidism), variable phosphate.
Hypophosphatemic rickets: normal calcium, low phosphate, normal PTH, normal bicarbonate.
Step 2: Compare this list against the given child.
The child has calcium $9$ mg/dl (normal), phosphate $2.4$ mg/dl (low), alkaline phosphatase $1040$ IU (high), PTH normal, bicarbonate normal.
Step 3: Match line by line.
Bicarbonate being normal removes both RTA types, since acidosis is their hallmark. Calcium being normal, not low, and PTH being normal, not raised, removes vitamin D dependent rickets, since that condition runs on hypocalcemia driving up PTH.
Step 4: Confirm the fit.
Only the hypophosphatemic rickets row matches every value: normal calcium, low phosphate, normal PTH, normal bicarbonate, with a raised alkaline phosphatase from active bone disease.
Step 5: State the answer.
The most probable diagnosis is Hypophosphatemic Rickets.
\[ \boxed{\text{Hypophosphatemic Rickets}} \]
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