**Core Concept**
Enzyme replacement therapy (ERT) is a medical treatment that involves replacing deficient enzymes in patients with inherited enzyme deficiencies. This approach is particularly beneficial for patients with lysosomal storage disorders, where deficient enzyme activity leads to the accumulation of toxic substances within cells.

**Why the Correct Answer is Right**
ERT works by administering recombinant forms of the deficient enzyme, which are then taken up by cells and facilitate the breakdown of accumulated substances. For instance, in the case of Gaucher disease, ERT involves the administration of recombinant glucocerebrosidase. This enzyme helps break down glucocerebroside, a key component of lysosomes in Gaucher disease patients. The use of ERT has significantly improved the quality of life for patients with Gaucher disease and other lysosomal storage disorders.

**Why Each Wrong Option is Incorrect**
**Option A:** Although gene therapy is another approach for treating genetic disorders, it is not classified as enzyme replacement therapy.
**Option B:** Hormone replacement therapy is a different treatment approach that involves replacing hormones in patients with hormone deficiencies.
**Option C:** Stem cell transplantation is a form of cell therapy that involves replacing damaged or diseased cells with healthy ones, but it is not the same as enzyme replacement therapy.

**Clinical Pearl / High-Yield Fact**
ERT is a highly effective treatment for patients with certain lysosomal storage disorders, but it is not without its challenges. Patients may experience immune reactions to the administered enzyme, which can lead to the formation of antibodies that neutralize the enzyme's activity.

**Correct Answer:** C. Pompe disease.