A 32-year-old Indian woman presents to the dermatology clinic with multiple pigmented lesions and severe photosensitivity that began in childhood. She reports freckling on sun-exposed areas and has had two episodes of skin cancer (basal cell carcinoma at age 28 and squamous cell carcinoma at age 31). Her parents are consanguineous. On examination, she has dry, atrophic skin with poikiloderma. Genetic testing reveals mutations in the XPA gene. Which of the following DNA repair mechanisms is primarily defective in this patient?