**Core Concept**
Enzyme replacement therapy (ERT) is a treatment approach that involves replacing deficient or missing enzymes in patients with genetic disorders, such as lysosomal storage diseases. This therapy aims to restore normal enzyme function, thereby alleviating symptoms and improving quality of life.

**Why the Correct Answer is Right**
ERT is available for several lysosomal storage diseases, including Fabry disease, Gaucher disease, and Pompe disease. However, some genetic disorders do not have a commercially available ERT. One notable exception is **Hunter syndrome**, a rare genetic disorder caused by a deficiency of the enzyme iduronate-2-sulfatase. Although research is ongoing to develop ERT for Hunter syndrome, it is not currently available.

**Why Each Wrong Option is Incorrect**
* **Option A:** Not applicable, as we are looking for a specific disorder without ERT.
* **Option B:** This option might be tempting, but it is incorrect. ERT is available for several lysosomal storage diseases, and this option does not specify a disorder without ERT.
* **Option C:** This option might also seem plausible, but it is incorrect. While there may be some challenges in developing ERT for certain disorders, this option does not accurately represent a specific condition without ERT.

**Clinical Pearl / High-Yield Fact**
When considering enzyme replacement therapy, it is essential to remember that not all genetic disorders have a commercially available ERT. Healthcare providers must carefully evaluate each patient's condition and discuss the available treatment options with them.

**Correct Answer: D. Hunter syndrome.**