**Core Concept**
Apert syndrome is a rare genetic disorder characterized by premature fusion of the skull bones, leading to craniosynostosis, and associated with abnormalities in the hands and feet. This condition is caused by mutations in the FGFR2 gene, which plays a crucial role in bone development and growth.

**Why the Correct Answer is Right**
Apert syndrome is a result of mutations in the FGFR2 gene, specifically in the exon IIIa region. This mutation leads to the formation of an abnormal protein that causes premature fusion of the skull bones. The FGFR2 gene encodes for a receptor tyrosine kinase that is involved in the signaling pathway that regulates bone growth and development. In Apert syndrome, the abnormal protein disrupts this signaling pathway, resulting in craniosynostosis and other associated abnormalities.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Apert syndrome is not caused by mutations in the FGFR1 gene, which is a different gene that also encodes for a receptor tyrosine kinase involved in bone development.

**Option B:** This option is incorrect because Crouzon syndrome is a different genetic disorder that is also characterized by craniosynostosis, but it is caused by mutations in the FGFR2 gene, specifically in the exon IIIc region.

**Option C:** This option is incorrect because Pfeiffer syndrome is a different genetic disorder that is also characterized by craniosynostosis, but it is caused by mutations in the FGFR1 or FGFR2 genes.

**Option D:** This option is incorrect because Saethre-Chotzen syndrome is a different genetic disorder that is characterized by craniosynostosis, but it is caused by mutations in the TWIST1 gene.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that Apert syndrome is a specific genetic disorder that is caused by mutations in the FGFR2 gene, and it is characterized by premature fusion of the skull bones and associated abnormalities in the hands and feet.

**Correct Answer:** D.