A Novel <scp><i>PHEX</i></scp> Mutation in X‐Linked Hypophosphataemic Rickets With Reduced Expression of <scp>NaPi</scp>‐<scp>IIa</scp> and <scp>NaPi</scp>‐<scp>IIc</scp> in the Proximal Tubules: A Case Report

X-linked hypophosphataemia (XLH, MIM#307800) is an inherited form of rickets resulting from mutations in the phosphate-regulating neutral endopeptidase (PHEX) gene on the X chromosome. These mutations lead to elevated circulating fibroblast growth factor 23 (FGF23), which disrupts phosphate homeostasis and contributes to XLH pathogenesis. We present a sporadic case of a 41-year-old woman diagnosed with rickets in childhood who later developed persistent proteinuria. Kidney biopsy revealed segmental sclerosis with a perihilar lesion in one of 19 glomeruli, along with dilated proximal tubules, reduced expression of the sodium-dependent phosphate transporters (NaPi-IIa and NaPi-IIc) and lysosomal particle accumulation in proximal tubule epithelial cells. Next-generation sequencing identified a novel heterozygous missense mutation in PHEX (c.2179T>A; p.Phe727Ile), which, to our knowledge, has not been previously reported. Detailed kidney biopsy findings in XLH are rare. This case report provides novel insights into XLH pathophysiology, highlighting kidney-specific pathological changes and reinforcing the importance of genetic testing for precise diagnosis and management.