**Core Concept**
Enzyme replacement therapy (ERT) is a pharmacological approach that involves the administration of a deficient enzyme to patients with genetic disorders characterized by enzyme deficiencies. This treatment aims to alleviate symptoms and improve the quality of life for patients with such conditions.

**Why the Correct Answer is Right**
ERT is specifically designed for disorders caused by enzyme deficiencies, where the body's cells are unable to produce sufficient amounts of the required enzyme. For instance, in the case of Gaucher's disease, patients lack the enzyme glucocerebrosidase, which is necessary for the breakdown of certain lipids. ERT involves the administration of recombinant glucocerebrosidase, thereby replacing the deficient enzyme and facilitating the breakdown of lipids.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because enzyme replacement therapy is not typically used for the treatment of metabolic disorders caused by mitochondrial dysfunction, such as Leigh's disease. Mitochondrial disorders often require different therapeutic approaches.

**Option B:** This option is incorrect because ERT is not a standard treatment for autoimmune disorders like rheumatoid arthritis. Instead, treatments for autoimmune disorders usually involve immunosuppressive medications or biologics that target specific components of the immune response.

**Option C:** This option is incorrect because ERT is not typically used for the treatment of disorders caused by receptor deficiencies, such as congenital adrenal hyperplasia. These conditions often require hormone replacement therapy or other targeted treatments.

**Clinical Pearl / High-Yield Fact**
ERT is a valuable treatment option for patients with genetic disorders caused by enzyme deficiencies, but it is essential to identify the specific enzyme deficiency and choose the correct enzyme replacement therapy.

**Correct Answer: A. Gaucher's disease**