Xeroderma pigmentosum is caused by defective nucleotide excision repair (NER). The specific step that is deficient is:

• A Recognition and removal of mismatched bases by MutS/MutL homologues
• B Rejoining of single-strand breaks by DNA ligase I
• C Removal of uracil from DNA by uracil DNA glycosylase
• D Recognition of bulky DNA adducts (UV-induced pyrimidine dimers) by XPC/XPA complex and dual incision by XPF-ERCC1 and XPG endonucleases ✓

Explanation

NER removes bulky helix-distorting lesions (UV pyrimidine dimers, chemical adducts); XP proteins (XPA-XPG) are required for damage recognition, DNA unwinding (XPB/XPD helicases in TFIIH), dual incision creating a 25–30 nt oligonucleotide, and gap filling. Mismatch repair uses MutSα/MutLα (MLH1/MSH2) to correct replication errors. Uracil removal is base excision repair. Ligase I is common to multiple repair pathways.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.