**Core Concept**
The Familial Adenomatous Polyposis (FAP) gene is a tumor suppressor gene responsible for the development of hundreds to thousands of polyps in the colon and rectum, leading to an increased risk of colorectal cancer. FAP is an autosomal dominant disorder.

**Why the Correct Answer is Right**
The FAP gene, also known as APC (Adenomatous Polyposis Coli), is a crucial gene involved in the regulation of cell growth and division. Mutations in the APC gene lead to the accumulation of β-catenin in the nucleus, resulting in the activation of Wnt signaling pathway, which promotes cell proliferation and tumor formation. The APC gene is located on the long arm of chromosome 5 (5q21-q22). The identification of the APC gene on chromosome 5 has led to genetic testing for FAP, enabling early detection and prophylactic colectomy to prevent colorectal cancer.

**Why Each Wrong Option is Incorrect**
**Option B:** Chromosome 8 is not associated with FAP. Chromosome 8 is involved in various diseases, including diabetes and some forms of cancer, but not FAP.
**Option C:** Chromosome 15 is not linked to FAP. Chromosome 15 is associated with other genetic disorders, such as Prader-Willi syndrome, but not FAP.
**Option D:** Chromosome X is not the location of the FAP gene. Chromosome X is involved in various genetic conditions, such as hemophilia and some forms of cancer, but not FAP.

**Clinical Pearl / High-Yield Fact**
FAP is an autosomal dominant disorder, meaning a single copy of the mutated gene is sufficient to cause the condition. This highlights the importance of genetic counseling and testing for individuals with a family history of FAP.

**✓ Correct Answer: A. Chromosome 5**