Laboratory tests demonstrated elevated serum lactate, HbA1c, and high-sensitivity cardiac troponin T. Targeted sequencing identified a pathogenic m.3243A > G variant in the MT-TL1 gene with 55.6% heteroplasmy, confirming mitochondrial encephalomyopathy with cardiac involvement. The gene discussed is MT-TL1; the disease is mitochondrial encephalomyopathy.