**Core Concept**
Familial adenomatous polyposis (FAP) is a genetic disorder characterized by the development of numerous adenomatous polyps in the colon and rectum. This condition is inherited in an autosomal dominant pattern, and its progression to colorectal cancer is nearly certain if left untreated. The underlying genetic defect is a mutation in a tumor suppressor gene.

**Why the Correct Answer is Right**
The gene associated with FAP is the adenomatous polyposis coli (APC) gene, located on chromosome 5q21-22. The APC gene acts as a tumor suppressor by regulating the beta-catenin protein, a key component of the Wnt signaling pathway. In the absence of the APC protein, beta-catenin accumulates and triggers the expression of genes that promote cell proliferation and tumor formation. The APC gene mutation leads to a loss of its tumor suppressor function, resulting in the development of adenomatous polyps and an increased risk of colorectal cancer.

**Why Each Wrong Option is Incorrect**
**Option A:** Not applicable.
**Option B:** While the MLH1 gene is associated with Lynch syndrome, a different hereditary colorectal cancer syndrome, it is not the gene linked to FAP.
**Option C:** The TP53 gene is a tumor suppressor gene involved in various cancers, but it is not specifically associated with FAP.
**Option D:** The DCC gene is a tumor suppressor gene involved in colorectal cancer, but it is not the primary gene associated with FAP.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that FAP is an autosomal dominant disorder, meaning that a single mutation in one copy of the APC gene is sufficient to cause the condition. This knowledge is crucial for genetic counseling and risk assessment in families with a history of FAP.

**Correct Answer: C. APC gene.**