**Core Concept**
Fragile X syndrome is a genetic disorder caused by a mutation in the FMR1 gene on the X chromosome, leading to a deficiency of the fragile X mental retardation protein (FMRP). This protein plays a crucial role in brain development and function. The disorder is inherited in an X-linked recessive pattern, meaning it primarily affects males, who have only one X chromosome.

**Why the Correct Answer is Right**
The FMR1 gene is located on the long arm of the X chromosome (Xq27.3) and codes for the FMRP protein. Mutations in this gene, such as expansions of the CGG repeat, result in the silencing of the gene and a subsequent reduction or absence of FMRP. This deficiency leads to abnormal brain development, intellectual disability, and various physical and behavioral characteristics. Females can be carriers of the mutated gene but are less likely to express the disorder due to X-chromosome inactivation.

**Why Each Wrong Option is Incorrect**
* **Option A:** Duchenne muscular dystrophy is an X-linked recessive disorder caused by mutations in the dystrophin gene on the X chromosome. However, it is characterized by progressive muscle degeneration and weakness, primarily affecting boys.
* **Option B:** Huntington's disease is an autosomal dominant disorder caused by mutations in the Huntingtin gene on chromosome 4. It is characterized by progressive motor dysfunction, cognitive decline, and psychiatric symptoms.
* **Option C:** Sickle cell anemia is an autosomal recessive disorder caused by mutations in the HBB gene on chromosome 11. It is characterized by abnormal hemoglobin production, leading to red blood cell deformity and various clinical manifestations.

**Clinical Pearl / High-Yield Fact**
Fragile X syndrome is the most common cause of inherited intellectual disability and the most common genetic cause of autism spectrum disorder.

**Correct Answer:** C.