The mother of a 4-year-old child notes blood when laundering his underwear. Physical examination reveals a rectal mass. On proctoscopy, there is a smooth-surfaced, pedunculated, 1.5-cm polyp. It is excised and microscopically shows cystically dilated crypts filled with mucin and inflammatory debris, but no dysplasia. What is the most likely diagnosis?
Familial adenomatous polyposis Gardner syndrome Juvenile polyp Lynch syndrome