Question

A child presents with growth failure. Biochemical analysis shows normal Ca2+, normal PTH, reduced phosphate, and increased ALP. What is the diagnosis? 

• A. Nutritional rickets
• B. Hypophosphatemic rickets
• C. Type 1 VDDR
• D. Type 2 VDDR

Hint:

In cases of rickets with normal calcium and PTH, but low phosphate and high ALP, consider hypophosphatemic rickets. Phosphate deficiency is the hallmark.

Answer 1

The Correct Option is B

Step 1: Build the biochemical fingerprint.
Four values are handed to us: serum calcium is normal, PTH is normal, phosphate is low, and ALP is high. Read together as a single pattern rather than four separate numbers, this is the classic signature of a renal phosphate-wasting disorder.

Step 2: Use calcium and PTH to split the differential.
In any vitamin-D-related rickets (nutritional deficiency, Type 1 VDDR, Type 2 VDDR), the chain is: low active vitamin D → reduced gut calcium absorption → tendency to hypocalcaemia → the parathyroids respond with secondary hyperparathyroidism. So in those conditions PTH is HIGH and calcium is low-to-low-normal. Here PTH is squarely normal and calcium is normal, which argues strongly AGAINST a vitamin-D-axis cause and points toward an isolated phosphate problem.

Step 3: Explain the low phosphate and high ALP.
If calcium handling is intact but phosphate is being dumped in the urine (renal tubular phosphate wasting, classically X-linked via excess FGF23), serum phosphate falls. Phosphate is essential for mineralising the growth plate, so deficient mineralisation produces rickets, growth failure, and a brisk rise in ALP from osteoblasts working overtime. That trio (normal Ca, normal PTH, low PO4, high ALP) maps onto hypophosphataemic rickets.

Step 4: Eliminate the vitamin-D options explicitly.
• Nutritional rickets → low calcium, HIGH PTH - excluded.
• Type 1 VDDR (1$\alpha$-hydroxylase defect) → low calcitriol, low calcium, HIGH PTH - excluded.
• Type 2 VDDR (receptor resistance) → high calcitriol but end-organ resistance, again with low calcium and HIGH PTH (often with alopecia) - excluded.
Only the phosphate-wasting picture leaves PTH and calcium untouched.

Final answer: The pattern of normal Ca$^{2+}$, normal PTH, low phosphate, and raised ALP fits Hypophosphatemic rickets (Option 2).