**Core Concept:** Fragile X syndrome is a genetic disorder caused by an expansion of guanine-cytosine (GC) repeats in the FMR1 gene on the X chromosome. It results in intellectual disability, autism spectrum disorders, and various other symptoms. The fragile X chromosome is a fragile site, which becomes visible under special staining techniques in the interphase nucleus during cell division.

**Why the Correct Answer is Right:** Option D is incorrect because the fragile X chromosome is a characteristic feature of fragile X syndrome, making it a diagnostic clue for the condition. Options A, B, and C are incorrect because they are either true aspects of the syndrome or are not directly related to FMR1 gene expansion or the fragile X chromosome.

**Why Each Wrong Option is Incorrect:**

A. **"Autism is not a feature":** Autism is a common feature of fragile X syndrome, along with intellectual disability. It is not a false aspect of the syndrome.

B. **"FMR1 gene expansion is not significant":** The FMR1 gene expansion is the primary genetic cause of fragile X syndrome. It leads to the production of an unstable RNA molecule, which results in reduced or absent protein function. This protein is involved in synaptic plasticity and development, leading to the characteristic features of the syndrome.

C. **"Intellectual disability is not present":** Intellectual disability is a key feature of fragile X syndrome, affecting cognitive development and overall intellectual functioning.

**Clinical Pearl:** Fragile X syndrome is a prime example of how a specific genetic mutation can lead to a range of clinical features due to its effects on cellular processes and molecular pathways. It highlights the importance of genetic testing in understanding and diagnosing these conditions.

**Correct Answer:** .