A 28-year-old Indian man is referred to the gastroenterology clinic for evaluation of chronic diarrhea and recurrent infections. He has a history of early-onset colorectal cancer (diagnosed at age 35 in his father, age 40 in his paternal uncle). On genetic counseling, he tests positive for a heterozygous mutation in the MLH1 gene. Colonoscopy reveals multiple adenomatous polyps. Which of the following DNA repair defects best explains his predisposition to colorectal cancer?