A 28-year-old Indian man presents to the gastroenterology clinic with a 2-year history of chronic diarrhea, weight loss, and recurrent infections. Colonoscopy reveals multiple colorectal polyps. His 45-year-old father died of colorectal cancer at age 50. Genetic counseling reveals a pathogenic mutation in the MLH1 gene. Which of the following DNA repair defects best explains the increased cancer predisposition in this family?