## Why Chromosomal Instability (CIN) pathway is right The annular "apple-core" lesion marked **A** is the classic presentation of left-sided colorectal adenocarcinoma arising via the adenoma-carcinoma sequence. This sequence occurs in approximately 80% of sporadic colorectal cancers and follows the CIN pathway: APC tumor suppressor mutation (5q21) initiates adenoma formation, followed by K-RAS activation (12p) promoting growth, SMAD4/DCC loss (18q) enabling invasion, and TP53 mutation (17p) driving malignant transformation. This stepwise accumulation of mutations over 10–15 years is the predominant molecular mechanism in sporadic CRC and familial adenomatous polyposis (FAP). Sabiston and Robbins both emphasize this as the canonical pathway for most colorectal carcinomas. ## Why each distractor is wrong - **Microsatellite instability (MSI) pathway**: While MSI accounts for ~15% of sporadic CRC (often due to MLH1 promoter hypermethylation) and is the hallmark of Lynch syndrome, it is NOT the predominant pathway in 80% of cases. MSI-high tumors tend to arise from sessile serrated polyps and have different clinical behavior. - **Serrated pathway**: This accounts for approximately 10% of CRC and is characterized by BRAF mutations and CpG island methylation. It is a distinct, minority pathway and does not explain the majority of adenoma-carcinoma sequence lesions. - **CpG island methylator phenotype (CIMP)**: While CIMP is an epigenetic mechanism that can overlap with MSI or serrated pathways, it is not the primary driver of the classic adenoma-carcinoma sequence and does not account for 80% of sporadic CRC. **High-Yield:** Apple-core left-sided CRC → adenoma-carcinoma sequence → CIN pathway (APC → K-RAS → DCC/SMAD4 → TP53) in 80% of sporadic cases. [cite: Sabiston Textbook of Surgery 21e Ch 51; Robbins 10e Ch 17; Schwartz 11e Ch 29]
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