Before prescribing cetuximab for metastatic colorectal carcinoma, which laboratory finding predicts lack of benefit and must be excluded?
- A HER2 amplification
- B KRAS mutation in codon 12 or 13 of exon 2 ✓
- C Microsatellite instability high status
- D BRAF V600E germline carriage
Correct answer: B. KRAS mutation in codon 12 or 13 of exon 2
Explanation
Cetuximab blocks EGFR, but downstream signalling through mutated KRAS remains constitutively active regardless of upstream receptor blockade, so tumours with activating KRAS mutations in exon 2 codons 12 or 13 do not respond. Testing for KRAS mutation status is mandatory before use. BRAF V600E is somatic, not germline, and while it worsens prognosis it does not serve as the standard exclusion test the way KRAS does.
Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.