**Core Concept**
Fragile X syndrome is a genetic disorder caused by an expansion of the CGG repeat in the FMR1 gene, leading to the silencing of the gene and the absence of the fragile X mental retardation protein (FMRP). This results in a range of neurodevelopmental and physical abnormalities.

**Why the Correct Answer is Right**
Fragile X syndrome is the most common cause of inherited intellectual disability, and the expansion of the CGG repeat is a hallmark of the disorder. The absence of FMRP disrupts synaptic plasticity and leads to abnormal neuronal function, contributing to the cognitive and behavioral symptoms associated with the disorder.

**Why Each Wrong Option is Incorrect**
* **Option A:** Fragile X syndrome is not caused by an autosomal dominant mutation. The expansion of the CGG repeat in the FMR1 gene is an autosomal dominant mutation, but it is not a simple gain-of-function mutation.
* **Option B:** Fragile X syndrome is not primarily associated with a deficiency of the protein fragile X mental retardation protein (FMRP). The disorder is actually characterized by the absence of FMRP due to the expansion of the CGG repeat.
* **Option C:** Fragile X syndrome is not typically associated with a loss of function in the FMR1 gene. The expansion of the CGG repeat leads to the silencing of the gene and the absence of FMRP.
* **Option D:** Fragile X syndrome is not a rare disorder. It is actually the most common cause of inherited intellectual disability, affecting approximately 1 in 4,000 males and 1 in 8,000 females.

**Clinical Pearl / High-Yield Fact**
The fragile X syndrome is often associated with a characteristic physical appearance, including a long and narrow face, a prominent jaw, and a large head size. This physical appearance is due to the absence of FMRP and the resulting abnormal neuronal development.

**Correct Answer: A. Fragile X syndrome is not caused by an autosomal dominant mutation.**