Infantile Fructose-1,6-Bisphosphatase Deficiency Masquerading as Mitochondriopathy.

Fructose-1,6-bisphosphatase 1 (FBP1) deficiency is a rare autosomal recessive disorder of gluconeogenesis. Affected children present with severe hypoglycemia and lactic acidosis in infancy. We report a case of a female child, aged one year and six months, born out of a third-degree consanguineous marriage, who initially presented with sudden-onset vomiting episodes and failure to thrive. Despite a clinical suspicion of mitochondrial disorder, biochemical investigations revealed elevated levels of alanine, glycine, lactic acid, pyruvic acid, 3-hydroxy isovaleric acid, fumaric acid, and 4-hydroxy phenylacetic acid. Clinical exome sequencing confirmed homozygous inheritance of a mutated gene, establishing the diagnosis of FBP1 deficiency. Differential diagnoses included mitochondrial disorders and transaldolase deficiency, but comprehensive genetic testing excluded these conditions. Management focused on dietary adjustments to avoid simple sugars and increase complex carbohydrates during illness. This case underscores the complexity of diagnosing rare metabolic disorders and highlights the pivotal role of genetic testing in accurate diagnosis and management.