A Tarui Disease Phenotype with Compensated Hemolysis and a Homozygous PFKM Variant of Uncertain Significance Mimicking Chronic Myelomonocytic Leukemia.

Background and Clinical Significance: Tarui disease, or glycogen storage disease type VII, is a rare autosomal recessive metabolic myopathy caused by muscle phosphofructokinase deficiency. Its manifestations include exercise intolerance, exertional myalgia, muscle cramps, myoglobinuria, rhabdomyolysis, hyperuricemia, and compensated hemolysis. The hematologic phenotype may obscure the underlying metabolic disorder and raise concern for a clonal myeloid neoplasm. Case Presentation: A 23-year-old man was referred for persistent mild thrombocytopenia following evaluation for jaundice and hepatosplenomegaly. He had undergone cholecystectomy at 18 years of age and reported exercise-induced myalgia, muscle cramps, and episodes of dark urine. Laboratory investigations demonstrated mild monocytosis, reticulocytosis, thrombocytopenia, hyperuricemia, elevated lactate dehydrogenase, and predominantly unconjugated hyperbilirubinemia, with a negative direct antiglobulin test. Selected inherited hemolytic disorders, hemoglobinopathies, and paroxysmal nocturnal hemoglobinuria were excluded. Bone marrow examination showed marked erythroid hyperplasia and mild megakaryocytic dysplasia. Testing for JAK2, CALR, and MPL mutations and an extended myeloid next-generation sequencing panel identified no pathogenic variants, and monocytosis resolved during follow-up. Whole-exome sequencing identified a homozygous PFKM missense variant, NM_001354735.1:c.1087A>T, p.(Ile363Phe), classified as a variant of uncertain significance. The patient subsequently developed severe rhabdomyolysis, with a creatine kinase level of 225,000 U/L and recovered after intensive intravenous hydration without renal impairment. Conclusions: Tarui disease should be considered in young patients with compensated hemolysis, hyperuricemia, exertional muscle symptoms, dark urine, or rhabdomyolysis, even when hematologic abnormalities suggest a myeloid disorder. The highly concordant phenotype and homozygous PFKM variant support a clinically probable diagnosis, although pathogenicity remains unconfirmed. Functional and segregation evidence may strengthen causal interpretation and support future variant reclassification.