**Question:** Mitochondrial DNA linked disease is

A. Leber's Hereditary Optic Neuropathy
B. Kearns-Sayre Syndrome
C. Myotonic Dystrophy
D. Wilson's Disease

**Correct Answer:** B. Kearns-Sayre Syndrome

**Core Concept:** Mitochondrial DNA (mtDNA) is a type of genetic material found within the mitochondria, the powerhouses of the cell responsible for producing energy in the form of ATP (adenosine triphosphate). Mitochondrial disorders are caused by mutations in mtDNA, leading to impaired mitochondrial function and energy production.

**Why the Correct Answer is Right:** Kearns-Sayre Syndrome is a mitochondrial DNA-linked disease caused by mutations in mtDNA. It is characterized by chronic progressive external ophthalmoplegia (CPEO), muscle weakness, and pigmentary degeneration of the retina. The disease is named after the two physicians who first described it, William Kearns and John Sayre.

**Why Each Wrong Option is Incorrect:**

A. Leber's Hereditary Optic Neuropathy (LHON) is a mitochondrial DNA disease, but it primarily affects the optic nerve and vision, not the muscles and skin as in Kearns-Sayre Syndrome.

B. Kearns-Sayre Syndrome and LHON are both mitochondrial DNA disorders, but they have distinct clinical features. Kearns-Sayre Syndrome involves muscle weakness and skin pigmentation changes, while LHON primarily affects the optic nerve and vision.

C. Myotonic Dystrophy is a disease characterized by muscle weakness caused by mutations in nuclear DNA, not mitochondrial DNA.

D. Wilson's Disease is a copper-related disorder causing liver dysfunction and neurological symptoms, not the muscle weakness and skin changes seen in Kearns-Sayre Syndrome.

**Clinical Pearl:** Mitochondrial disorders can present with a wide range of symptoms depending on the affected organ systems. A thorough clinical assessment, including neurological and ophthalmological examinations, is crucial for diagnosis and differential diagnosis.