Episode five closes the chapter on the other half of why colonoscopy quality matters, the serrated pathway where missed right-sided lesions become interval cancer. Sessile serrated lesions run through BRAF V600E, CIMP-high hypermethylation, MLH1 silencing, and microsatellite instability, which is why sessile serrated lesion detection rate is its own quality metric. That molecular route drives the Lynch reflex tree: mismatch repair immunohistochemistry, then BRAF and MLH1 methylation to triage sporadic disease, with the other loss patterns going straight to germline testing because sporadic biology cannot explain them. The special populations then bend the standard algorithm one mechanism at a time, as the kidneys forbid phosphate, the liver forbids morphine, the fetus forbids first-trimester benzodiazepines, and the IBD colon demands chromoendoscopy. Topics covered Serrated pathway molecular biology Lynch universal tumor screening BRAF V600E as sporadic discriminator MMR loss patterns and germline testing Serrated polyposis syndrome criteria Physiology-driven special populations Pregnancy, cirrhosis, and ESRD colonoscopy IBD chromoendoscopy surveillance Periprocedural antithrombotic management Key decisions Combined MLH1 and PMS2 loss on immunohistochemistry reflexes to BRAF V600E testing, and a positive result triages the patient away from germline testing because the cancer is sporadic CIMP-pathway disease. A negative BRAF result reflexes to MLH1 promoter methylation testing, and only when both BRAF and methylation are negative does germline sequencing for Lynch syndrome become indicated. Isolated loss of MSH2, MSH6, or PMS2 goes straight to germline testing without BRAF or methylation, because sporadic methylation acts on MLH1 specifically and cannot explain those losses. The WHO twenty nineteen serrated polyposis definition requires either five or more serrated polyps proximal to the rectum all at least five millimeters with two over ten, or more than twenty serrated polyps with at least five proximal to the rectum. Elective colonoscopy in pregnancy is deferred when possible, done in the second trimester when necessary, with propofol preferred, first-trimester benzodiazepines avoided, and left lateral decubitus positioning to protect venous return. Sodium phosphate preparations are contraindicated in chronic kidney disease, on diuretics or ACE inhibitors, in hypertension, and in the elderly, with iso-osmotic PEG-electrolyte the standard alternative and hemodialysis timed to the morning after dialysis. Elective polypectomy on full dual antiplatelet therapy after a recent stent is deferred until DAPT can be safely de-escalated, and IBD dysplasia surveillance uses chromoendoscopy with targeted biopsies rather than random four-quadrant sampling. For the full chapter with MCQs, tables, and primary-guideline references, visit www.boardpearls.com. Questions or feedback: [email protected].

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