An 8-year-old boy presented with a skin tumor on his left cheek. He had always avoided exposure to sunlight because it made his skin blister. His skin had scattered areas of hyperpigmentation. He mostly have:

Xeroderma pigmentosum (XP) is a rare autosomal recessive condition in which the mechanisms for repair of DNA subsequent to damage by UV irradiation are defective. This arises because of mutations in the genes encoding components of the nucleotide excision pathway of DNA repair. Ref: Murray R.K., Grossc P.L. (2011). Chapter 57. Biochemical Case Histories. In D.A. Bender, K.M. Botham, P.A. Weil, P.J. Kennelly, R.K. Murray, V.W. Rodwell (Eds), Harper's Illustrated Biochemistry, 29e.