A puzzling renal Fanconi syndrome.

Renal Fanconi syndrome (FS) can be either acquired or inherited. When FS presents at a young age, it is typically inherited, with cystinosis being the most common cause. In this report we describe a rare cause of autosomal dominant Fanconi syndrome, Fanconi renotubular syndrome type 3 (FRTS3), caused by the already reported heterozygous p.E3K variant in the gene. Only two FRTS3 families have been reported in the literature, and the kidney function was stated as normal or only slightly decreased into late life. Our family expands the spectrum of FRTS3, with some individuals showing only glucosuria and mild low-molecular-weight proteinuria, while others exhibited complete Fanconi syndrome with rickets. Importantly, we observed impairment of kidney function at a young age in our proband, highlighting a broader phenotypic variability associated with FRTS3.