An 8-year-old with genu valgum: Answers

Excerpt

X-ray findings of knee (Fig. 1a, b) and wrist (Fig. 1c) are suggestive of rickets. His laboratory findings showed normal calcium, but low phosphorous, which is suggestive of hypophosphatemic rickets. Differential diagnosis of hypophosphatemic rickets can be further differentiated by alkaline phosphatase activity. Low alkaline phosphatase would suggest hypophosphatasia; however, our patient had high alkaline phosphatase. Hypophosphatemic rickets with increased fibroblast growth factor 23 (FGF23) could be due to X-linked, autosomal dominant, or recessive hypophosphatemic rickets. Since FGF23 levels are not widely available, 1,25-dihydroxyvitamin D, which is suppressed by FGF23, could give us a clue regarding this condition. If the patient had an elevated 1,25-dihydroxyvitamin D level, along with hypophosphatemic rickets, then the assumption would be that FGF23 is normal and differential diagnosis would include hypophosphatemic rickets with hypercalciuria, Dent’s disease, or renal tubular disorders. Our patient did not have a proteinuria or renal Fanconi syndrome which would help exclude this diagnosis. This differential diagnosis is also summarized in Fig. 2.

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