Disorders of intracellular cobalamin metabolism are rare but treatable conditions that mimic bone marrow failure syndromes.

An eight-month-old male presented with macrocytic anemia, reticulocytopenia, neutropenia, infections, failure to thrive, and developmental delay. Bone marrow examination showed hypocellularity with paucity of myeloid precursors, dysplastic megakaryocytes, fibrosis, and cytoplasmic vacuolization of hematopoietic precursors. Whole-exome-sequencing identified a homozygous LMBRD1 variant (c.907C > A; p.Pro303Thr), confirmed by parental segregation. Treatment with parenteral hydroxocobalamin resulted in partial improvement.

LMBRD1-related cblF deficiency is an exceptionally rare but treatable mimic of inherited bone marrow failure. Early recognition enables targeted therapy.