Nijmegen Breakage Syndrome

A 5-year-old girl of Ashkenazi Slavic descent presents with severe microcephaly, growth failure, bird-like facies, and recurrent sinopulmonary infections since infancy. Immunologic evaluation reveals hypogammaglobulinemia with reduced T-cell counts and poor antibody responses. Karyotype of PHA-stimulated lymphocytes demonstrates the chromosomal abnormality marked **B** in the diagram. Unlike her cousin with ataxia-telangiectasia, she has no cerebellar ataxia or oculocutaneous telangiectasias, and her alpha-fetoprotein is normal. Genetic testing confirms a homozygous NBN gene mutation. Which of the following best explains the chromosomal instability pattern marked **B**?