How is familial adenomatous polyposis (FAP) diagnosed?

Familial adenomatous polyposis (FAP) is diagnosed primarily based on the identification of numerous adenomatous polyps in the colon and rectum. The correct answer, identifying the presence of more than 100 adenomas detected endoscopically, is essential for the diagnosis of FAP, as this condition is characterized by the development of hundreds to thousands of colorectal polyps, which virtually guarantees the progression to colorectal cancer if left untreated.

In contrast, a diagnosis based on fewer than 50 adenomas does not meet the criteria for FAP, as this number is significantly lower than what is typically observed in affected individuals. The requirement of polyps observed only after age 40 does not account for the earlier onset commonly associated with FAP, as symptoms and polyps can appear much earlier, often in the teenage years. Lastly, the identification of adenomas in a relative after age 50 lacks specificity and does not fulfill the criteria for diagnosing FAP, where a family history of polyposis and the presence of numerous polyps itself are critical for accurate diagnosis.

In summary, the diagnosis of familial adenomatous polyposis hinges on the detection of extensive polyps—specifically, more than 100 adenomas, making