If left untreated, what is the most likely lifetime risk of colon cancer in a patient with significant colonic abnormalities and a sister with similar findings?

The scenario describes a patient with significant colonic abnormalities and a sister with similar findings, which raises the suspicion of a hereditary colorectal cancer syndrome, such as Familial Adenomatous Polyposis (FAP) or Lynch syndrome (hereditary non-polyposis colorectal cancer).

In conditions like FAP, individuals develop hundreds to thousands of polyps in the colon and rectum at a young age, with an almost certain progression to colorectal cancer if left untreated, often before the age of 40. In the case of Lynch syndrome, individuals have a significantly increased risk of developing colorectal cancer, often in combination with other cancers.

Given the family history and the significant colonic abnormalities in this patient, the lifetime risk of developing colorectal cancer can approach nearly 100% without intervention. Screening, surveillance, and preventive measures, such as colectomy, can dramatically reduce this risk, but if left untreated, the risk remains extremely high.

The other options represent lower risks that would be more typical for sporadic cases of colon cancer, not situations involving hereditary syndromes with a strong familial link and significant abnormalities. Therefore, the lifetime risk of colon cancer in this patient is appropriately assessed as close to 100%.