## **Core Concept**
Familial Adenomatous Polyposis (FAP) is a rare, inherited condition characterized by the development of hundreds to thousands of polyps in the colon and rectum. It is caused by mutations in the **APC gene**, which is a tumor suppressor gene. The condition is defined by the presence of multiple adenomatous polyps in the colon.

## **Why the Correct Answer is Right**
The correct answer is **B. 100**. FAP is diagnosed when more than 100 adenomatous polyps are present in the colon. This condition is a significant risk factor for colorectal cancer, as the numerous polyps have a high potential for malignant transformation if left untreated. The presence of more than 100 polyps is a hallmark of FAP and distinguishes it from other polyposis syndromes.

## **Why Each Wrong Option is Incorrect**
- **Option A: 10** - This number is too low to be diagnostic of FAP. While FAP patients can have as few as 10-20 polyps at a young age, the diagnosis is generally confirmed when the polyp count exceeds 100.
- **Option C: 500** - Although patients with FAP can have hundreds to thousands of polyps, the diagnostic threshold is more than 100, not 500.
- **Option D: 1000** - Similar to option C, while many patients with FAP have over 1000 polyps, the defining criterion is having more than 100 polyps.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that FAP typically presents in the teenage years to early twenties, and **screening and prophylactic colectomy are crucial** to prevent colorectal cancer. Early diagnosis through genetic testing and colonoscopy is vital for management.

## **Correct Answer: B. 100**