Which of the following types of colonic polyps is associated with the highest incidence of malignant degeneration?

c. Villous adenoma

Adenomas can be divided into tubular (75% to 100% tubular component), tubulovillous (25% to 75% villous component), or villous (75% to 100% villous) adenomas. The most common type is tubular adenoma, or adenomatous polyp, which constitutes about 75% of neoplastic polyps. Tubulovillous adenomas represent 15% and pure villous adenomas 10% of neoplastic polyps. All adenomas contain some degree of dysplasia or cellular atypia. This dysplasia can be graded from mild to severe. Carcinoma in situ and severe dysplasia have been grouped together under the classification of high-grade dysplasia. In carcinoma in situ, there is no invasion into the muscularis mucosa, as there is in invasive carcinoma. The incidence of invasive malignancy differs markedly for the three types of adenomas and increases with size. In general, malignancies are seen in 5% of adenomatous polyps, in 22% of tubulovillous adenomas, and in 40% of villous lesions. Hence, although villous lesions are much less common, they are more likely to harbor a malignancy.