A 17-Month-old Boy With Pancytopenia Caused by a Rare Genetic Defect of Vitamin B12 Malabsorption.

Imerslund-Grasbeck syndrome is an autosomal recessive disorder of vitamin B12 malabsorption presenting with megaloblastic anemia and mild proteinuria in childhood. The disorder is caused by biallelic pathogenic variants in the CUBN or AMN genes, which encode proteins involved in B12 absorption. We present the case of a 17-month-old boy with failure to thrive, pancytopenia, and fevers. His megaloblastic anemia was overlooked leading to unnecessary invasive testing. Findings on bone marrow biopsy prompted investigation for genetic disorders of B12 metabolism. Exome sequencing uncovered 1 known pathogenic variant and 1 novel likely pathogenic variant in CUBN, confirming the diagnosis of Imerslund-Grasbeck syndrome.