Lynch Syndrome HNPCC Autosomal Dominant

A 45-year-old woman presents with a family history shown in the pedigree diagram marked **A**, which demonstrates autosomal dominant inheritance of early-onset colorectal and endometrial cancers across three generations. Her mother was diagnosed with colorectal cancer at age 38, and her maternal aunt with endometrial cancer at age 42. Genetic testing confirms a pathogenic variant in MLH1. Which of the following best describes the molecular basis of tumour development in this patient's Lynch syndrome?