**Core Concept**
Hunter disease, also known as mucopolysaccharidosis II (MPS II), is a rare genetic disorder caused by a deficiency of the enzyme iduronate-2-sulfatase (I2S). This enzyme is crucial for the breakdown of sugar molecules, leading to the accumulation of glycosaminoglycans (GAGs) in various tissues.

**Why the Correct Answer is Right**
The correct answer involves the X chromosome because Hunter disease is an X-linked recessive disorder. The gene encoding the iduronate-2-sulfatase enzyme is located on the X chromosome, and mutations in this gene lead to the production of a non-functional enzyme. As a result, the deficiency of iduronate-2-sulfatase causes the accumulation of GAGs, leading to the characteristic features of Hunter disease.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because Hunter disease is not caused by a defect in the chromosome 21, which is associated with Down syndrome.
* **Option B:** This option is incorrect because Hunter disease is not caused by a defect in the chromosome 6, which is associated with autoimmune disorders like lupus.
* **Option C:** This option is incorrect because Hunter disease is not caused by a defect in the chromosome 22, which is associated with other genetic disorders like tuberous sclerosis.

**Clinical Pearl / High-Yield Fact**
Hunter disease is a classic example of an X-linked recessive disorder, where affected males are more common than affected females due to the random X-chromosome inactivation in females.

**Correct Answer:** D.