Cell Cycle, Checkpoints, and Cyclins

A 28-year-old woman presents with a history of early-onset breast cancer (diagnosed at age 22) and a family history of multiple malignancies including sarcoma and adrenocortical carcinoma in childhood relatives. Genetic testing reveals a germline mutation in TP53. The checkpoint marked **A** in the diagram is compromised due to loss of p53 function. Which of the following best explains the increased cancer susceptibility in this patient?