LAUSR

A 5-year-old girl of Cook Island/Caucasian descent presented with short stature, progressive bowing leg deformities and pain that were limiting mobility and precluding age-appropriate activities such as climbing on playground equipment (Fig. 1). There was no family history of bone disorders or short stature. She had appropriate dietary calcium intake. Her height (3rd to 5th centile) was disproportionate to her weight (75th to 90th centile) and midparental height (90th centile). She had tender, widened lower limb metaphyses, moderate genu varus and bilateral tibial torsion (Fig. 1). No frontal bossing, rachitic rosary, dental findings or weakness were apparent. There was no history of frequent respiratory infections. She had received high-dose Vitamin D and calcium therapy from 3 years of age for a provisional diagnosis of Vitamin D and calcium deficiencies despite serum values within the agecorrected reference range (Table 1). She had hypophosphataemia and a low tubular maximal reabsorption of phosphate (TmP) adjusted for glomerular filtration rate (TmP/GFR). The intact Fibroblast Growth Factor 23 (FGF23) concentration was 55.1 pg/mL (<30 pg/mL). A massively parallel sequencing hereditary rickets panel revealed a heterozygous de novo pathogenic variant PHEX:c.349 + 1G > A, and a paternally inherited variant of unknown significance PHEX:c.922A > G p.Met308Val. With a clinically unaffected father, the latter variant was considered benign. A diagnosis of X-linked hypophosphataemic (XLH) rickets was made. She commenced oral calcitriol and phosphate with improvement in clinical, radiological and biochemical parameters.