combined oxidative phosphorylation defect type 9 (MONDO_0013811) is a rare mitochondrial disease due to a defect in mitochondrial protein synthesis characterized by initially normal growth and development followed by the infantile-onset of failure to thrive, psychomotor delay, poor feeding, dyspnea, severe hypertrophic cardiomyopathy and hepatomegaly. Laboratory studies report increased plasma lactate and alanine, abnormal liver enzymes and decreased activity of mitochondrial respiratory chain complexes I, III, IV, and V. Also known as: COXPD9, MRPL3 combined oxidative phosphorylation deficiency, combined oxidative phosphorylation deficiency caused by mutation in MRPL3, combined oxidative phosphorylation deficiency type 9.