**Core Concept**
X-linked dominant disorders are a group of genetic conditions caused by mutations in genes located on the X chromosome. These disorders are characterized by their dominant pattern of inheritance, where a single copy of the mutated gene is enough to cause the condition. X-linked dominant disorders often affect females, who have two X chromosomes, and males, who have one X and one Y chromosome.

**Why the Correct Answer is Right**
The correct answer is a specific X-linked dominant disorder. This disorder is caused by a mutation in the _GluR6_ gene, which codes for a subunit of the AMPA receptor. The mutation leads to abnormal glutamate receptor function, causing a range of symptoms including seizures, intellectual disability, and distinctive physical features. The disorder is characterized by its early onset, typically in infancy, and its severe progression.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is not an X-linked dominant disorder. X-linked recessive disorders, like hemophilia, affect males more frequently than females.
**Option B:** This option is incorrect because it is a chromosomal disorder caused by an extra copy of chromosome 21, not an X-linked dominant disorder.
**Option C:** This option is incorrect because it is a genetic disorder caused by a mutation in the _FMR1_ gene, which is located on the X chromosome but is an X-linked recessive disorder, not a dominant one.

**Clinical Pearl / High-Yield Fact**
X-linked dominant disorders often have a more severe impact on females than males, as females have two X chromosomes and can compensate for the mutated gene with the second X chromosome. However, males, who have only one X chromosome, are more likely to be affected by X-linked dominant disorders and often exhibit more severe symptoms.

**Correct Answer: C. Fragile X syndrome**