A tumor shows microsatellite instability (MSI-H) on molecular testing. Which DNA repair pathway is most likely defective?
- A Nucleotide excision repair
- B Base excision repair
- C Mismatch repair ✓
- D Homologous recombination repair
Correct answer: C. Mismatch repair
Explanation
Microsatellite instability results from defective mismatch repair (MMR) proteins — MLH1, MSH2, MSH6, PMS2. When MMR is deficient, replication errors at repetitive microsatellite sequences accumulate and are not corrected, expanding or contracting these repeats. This is the hallmark of Lynch syndrome and a subset of sporadic colorectal cancers.
Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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