Question

A child with this deformity. He has recurrent tooth abscesses. Calcium normal, Phosphorus low, PTH normal, ALP high. Diagnosis is?

• A. Nutritional rickets
• B. VDDR1
• C. VDDR2
• D. Hypophosphatemic rickets

Hint:

Normal calcium with normal PTH but low phosphate — plus recurrent dental abscesses — is a renal phosphate-wasting rickets, not a calcium/vitamin-D problem.

Answer 1

The Correct Option is D

Step 1: Read the biochemistry first. The decisive numbers are: Calcium normal, Phosphorus low, PTH normal, ALP high. Normal calcium with normal PTH but isolated low phosphate points to a primary renal phosphate-wasting disorder rather than a calcium/vitamin-D problem. The deformity (bowed legs / genu varum) plus a raised ALP confirms active rickets.

Step 2: Why Hypophosphatemic rickets (Option D) is correct. X-linked hypophosphatemic rickets (the commonest inherited rickets) is caused by excess FGF23, which makes the kidney waste phosphate. The pattern is therefore: low serum phosphate, normal serum calcium, normal (not raised) PTH, and high ALP from active bone turnover - an exact match. A characteristic clinical clue is recurrent dental/periapical abscesses due to defective dentine mineralisation and enlarged pulp chambers - again exactly as stated.

Step 3: Why the other options are wrong.
• Nutritional (vitamin-D deficiency) rickets (A): low vitamin D → low calcium → secondary hyperparathyroidism, so PTH would be high, not normal. PTH normal rules it out.
• VDDR type 1 (B): deficiency of 1α-hydroxylase → low calcitriol → low calcium → high PTH. Again PTH would be raised.
• VDDR type 2 (C): end-organ vitamin-D receptor resistance → low calcium → high PTH, classically with alopecia. The normal PTH here excludes it.
Only hypophosphatemic rickets keeps calcium AND PTH normal while phosphate is low.

Final answer: D (Hypophosphatemic rickets).