Mitochondrial dysfunction is a recognized cause of early-onset cardiomyopathy. Mutations in ELAC2, encoding mitochondrial RNase Z, disrupt mitochondrial mt-tRNA processing and impair oxidative phosphorylation, resulting in combined oxidative phosphorylation deficiency type 17. We report a 2-month-old female infant with a homozygous ELAC2 variant who presented with acute dilated cardiomyopathy, severe lactic acidosis, and rapid clinical deterioration. Notably, plasma proline was markedly elevated (>2500 ?mol/L), a finding not previously described in ELAC2-related disease. Despite intensive supportive therapy, the patient died within days of presentation. This case expands the phenotypic and biochemical spectrum of ELAC2-related disease and emphasizes the importance of detailed metabolic evaluation in infants presenting with unexplained cardiomyopathy.