Unraveling the mystery: Exploring a case of prolonged fever and hidden genetics: Case report.

This case report highlights an unusual presentation of transcobalamin 2 (TCN2) deficiency. It is a rare autosomal recessive disorder. In this case, we present a three-month-old child with prolonged fever and pancytopenia. Initial investigations could not reach a diagnosis. Subsequent investigations revealed normal serum vitamin B12 levels and elevated homocysteine levels, suggesting a vitamin B12 metabolic disorder. Genetic testing confirmed the presence of pathogenic variations in the transcobalamin 2 gene that led to establishing the diagnosis of transcobalamin 2 deficiency. The initiation of vitamin B12 supplementation led to the resolution of the patient's symptoms and the normalization of hematological parameters. Recognizing transcobalamin 2 deficiency as a possible cause of prolonged fever and low blood cell counts (pancytopenia) led to further specialized testing and genetic analysis. Considering this rare condition as a potential diagnosis, even when vitamin B12 levels appear normal is crucial. Comprehensive lab tests and genetic testing are essential to correctly identify these uncommon yet treatable disorders and guide appropriate treatment.