A 26-year-old man with a family history of colon carcinoma undergoes a surveillance colonoscopy. It reveals hundreds of polyps in the colon and two focal 0.5-cm ulcerated areas. A biopsy specimen from an ulcer reveals irregularly shaped glands that have penetrated into the muscular layer. Which of the following molecular events is believed to occur very early in the evolution of his colonic disease process?

## **Core Concept**
The patient's condition is suggestive of Familial Adenomatous Polyposis (FAP), a hereditary colorectal cancer syndrome characterized by the development of hundreds to thousands of polyps in the colon and rectum. FAP is caused by mutations in the **APC** (adenomatous polyposis coli) gene, which is a tumor suppressor gene. The progression from normal epithelium to colorectal cancer involves a series of molecular alterations.

## **Why the Correct Answer is Right**
The correct answer, **APC gene mutation**, is believed to occur very early in the evolution of FAP and colorectal cancer. The **APC gene** plays a crucial role in regulating cell growth and division by inhibiting the Wnt/β-catenin signaling pathway. Mutations in the **APC gene** lead to the accumulation of β-catenin, resulting in increased cell proliferation and polyp formation. This event is considered an initiating step in the adenoma-carcinoma sequence.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While **KRAS mutations** are common in colorectal cancers and occur relatively early in the adenoma-carcinoma sequence, they typically occur after **APC mutations**. **KRAS** encodes a protein involved in cell signal transduction, and its mutation leads to constitutive activation of downstream effectors, promoting cell growth and survival.
- **Option B:** **TP53 mutations** are associated with more advanced lesions and are involved in the transition from adenoma to carcinoma. **TP53** is a tumor suppressor gene that encodes a transcription factor crucial for regulating cell cycle, apoptosis, and genomic stability.
- **Option D:** **BRAF mutations** are also found in colorectal cancers, particularly in those with microsatellite instability (MSI), such as serrated adenocarcinomas. While **BRAF mutations** are important in certain colorectal cancer subtypes, they are not the earliest event in the FAP-related adenoma-carcinoma sequence.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that patients with FAP often undergo prophylactic colectomy due to the high risk of colorectal cancer, given the large number of adenomatous polyps that inevitably develop. Early diagnosis through genetic testing can significantly impact management and outcomes.

## **Correct Answer:** .

✓ Correct Answer: A. Activation of the WNT signaling pathway