Cellular energy is primarily derived from mitochondrial respiration. The tricarboxylic acid (TCA) cycle is an essential part of this process, oxidizing nutrients and producing reducing equivalents to feed mitochondrial oxidative phosphorylation. Defects in genes encoding TCA cycle enzymes can result in rare, debilitating diseases with multi-organ manifestations. One such disease is fumarate hydratase (FH) deficiency, an autosomal recessive disorder caused by a mutation in the FH gene, which encodes the fumarase enzyme responsible for hydrolyzing fumarate to malate in the TCA cycle. The disease causes severe neurological and developmental impairment and onsets at infancy. Accurate and timely diagnosis is difficult due to the disease's rarity, phenotypic overlap with other metabolic disorders, and the lack of existing clinical assays for FH. Although current clinical diagnosis relies on detecting elevated urine fumarate, an atypical case with normal urine fumarate but classic FH disease symptoms suggests the need for more specific diagnostic methods. This study aims to identify other biochemical markers to improve diagnostic accuracy by analyzing enzyme activity and respirometry.