What condition is characterized by a 100% risk of colorectal cancer if untreated?

Familial adenomatous polyposis (FAP) is indeed characterized by a near 100% risk of developing colorectal cancer if left untreated. This autosomal dominant condition is marked by the development of numerous adenomatous polyps in the colon and rectum, typically starting in adolescence or early adulthood.

Individuals with FAP may develop thousands of polyps which, over time, have a high likelihood of progressing to colorectal cancer. The mechanism involves mutations in the APC gene, which is crucial for regulating cell growth and development in the intestinal epithelium. The accumulation of mutations in the polyps eventually leads to malignancy. Due to the overwhelming risk, affected individuals are often recommended to undergo prophylactic colectomy, usually in their late teens or early twenties, to prevent the onset of colorectal cancer.

In contrast, Lynch syndrome (formerly HNPCC) carries a significant risk of colorectal cancer but is not as absolute. Inflammatory bowel disease (IBD) and Peutz-Jeghers syndrome also have increased risks for colorectal cancer, but these risks vary and are not comparably definitive as those associated with FAP.