Bloom Syndrome — Photosensitive Malar Rash

A 5-year-old child of Ashkenazi Jewish descent presents with profound short stature (below 1st centile), low birth weight, and a distinctive photosensitive facial rash that appeared in infancy. The rash shown as **B** in the diagram is characterized by erythema, telangiectasia, and scarring across the malar area and bridge of nose following sun exposure, with sparing of shaded areas. Cytogenetic studies reveal markedly increased sister chromatid exchanges (SCE) and quadriradial configurations. Which of the following best characterizes the underlying genetic defect responsible for the rash marked **B**?