Benign proximal tubular albuminuria due to AMN mutation: A challenging presentation of Imerslund-Gräsbeck syndrome.

A 3-year-old boy presented with dark-colored urine for 4 months. His history was negative for infections, but he was taking oral methylcobalamin treatment for a persistent deficiency. His parents were first-degree cousins, and a female cousin had proteinuria of unknown etiology. A physical examination and laboratory examination revealed no abnormalities except for non-orthostatic nephritic proteinuria and low levels of vitamin B12. Albumin was the main protein in the urine. Kidney biopsy showed nonspecific changes. Genetic analysis identified a homozygous pathogenic AMN mutation, confirming Imerslund-Grâsbeck syndrome (IGS). Angiotensin-converting enzyme inhibitor was prescribed but discontinued due to stable protein levels. After 4 years, kidney function remained stable. Imerslund-Grâsbeck syndrome is a rare autosomal recessive disorder that affects vitamin B12 and protein, particularly albumin absorption. While typically presenting with megaloblastic anemia, AMN mutations show variable phenotypes. Proteinuria is resistant to ACE inhibitors, and currently, there is no specific treatment.