## **Core Concept**
Leigh disease, also known as subacute necrotizing encephalomyelopathy, is a severe neurological disorder that usually becomes apparent in the first year of life. It is characterized by progressive loss of mental and motor skills (psychomotor regression) and typically results from mutations in mitochondrial DNA. The disorder is associated with abnormalities in energy production, particularly within the mitochondria.

## **Why the Correct Answer is Right**
The correct answer, **Pyruvate and lactate**, is related to the underlying biochemical defect in Leigh disease. This disorder often results from defects in the mitochondrial respiratory chain, particularly complexes I, II, III, IV, and V. One of the hallmark biochemical abnormalities is the accumulation of pyruvate and lactate due to the impaired conversion of pyruvate to acetyl-CoA, a critical step that occurs within the mitochondria. This impairment leads to elevated levels of pyruvate and lactate in the blood and cerebrospinal fluid.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While certain organic acidemias can lead to accumulation of organic acids, this option is too vague and does not directly relate to the primary biochemical defect seen in Leigh disease.
- **Option B:** This option might relate to disorders of fatty acid oxidation or other metabolic pathways but does not specifically pertain to the primary defect observed in Leigh disease.
- **Option D:** Similarly, this option does not directly relate to the known biochemical abnormalities associated with Leigh disease.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl for Leigh disease is that it often presents with a distinctive pattern of brain lesions visible on MRI, particularly in the basal ganglia, thalamus, and brainstem. Early diagnosis is critical, as some forms of Leigh disease may be treated with coenzyme Q10 or other supportive therapies, although these do not reverse existing damage.

## **Correct Answer:** . Pyruvate and lactate