A an autosomal recessive caused by pathogenic variants in the SDHD gene, leading to dysfunction of mitochondrial complex II. Clinical features are variable and may include Leigh syndrome, cardiomyopathy, and other neurological and muscular manifestations. The disease is MONDO_0030937 (mitochondrial complex 2 deficiency, nuclear type 3). Also known as: MC2DN3, SDHD-related Nuclear type mitochondrial complex II deficiency, mitochondrial complex II deficiency, nuclear type 3.