Colon and Rectum

Polyps and colorectal cancer (sporadic + hereditary), inflammatory and infectious colitides, volvulus, diverticular disease, obstruction/pseudo-obstruction, and lower GI bleeding.

High-Yield Points

• Vascular supply: SMA → ascending + proximal ⅔ transverse (ileocolic, right, middle colic); IMA → distal ⅓ transverse, descending, sigmoid, upper rectum (left colic, sigmoid, superior rectal). Watershed areas = splenic flexure (Griffith's point) and rectosigmoid (Sudeck's point) → ischemic colitis. Rectum has dual supply (superior rectal from IMA; middle/inferior from internal iliac), so it resists ischemia and has no diverticula (taeniae splay out).
• Polyps: hyperplastic = most common, non-neoplastic. Adenomatous = neoplastic; malignancy risk tubular (<5%) < tubulovillous (20%) < villous (40%). Cancer risk ↑ with villous histology, size >1 cm, high-grade dysplasia. Pedunculated polyp with favorable invasive focus (clear margin, no LVI, well-diff) → polypectomy + surveillance; sessile with invasion → colectomy.
• CRC = 2nd leading cause of cancer death. APC → KRAS → p53 adenoma-carcinoma sequence. Screening: colonoscopy at 50 (average risk), age 40 or 10 yr before a first-degree relative's diagnosis. Most common site = sigmoid. Nodal status = most important prognostic factor. Associations: Strep bovis (gallolyticus) endocarditis & Clostridium septicum.
• CRC spread/Tx: liver (#1, via portal vein) then lung — resect isolated liver/lung mets. Oncologic resection = 5 cm margins + ≥12 nodes. Adjuvant chemo (FOLFOX) for stage III/IV colon. KRAS-mutant → no benefit from cetuximab.
• Rectal cancer: stage with pelvic MRI (+ EUS for T/N); total mesorectal excision lowers recurrence. Stage II–III rectal → neoadjuvant chemoradiation, then surgery. APR (permanent colostomy) for tumors <2 cm from anal verge / sphincter involvement; LAR for upper/mid rectum.
• Hereditary CRC: FAP (AD, APC 5q) → thousands of polyps, 100% cancer by ~40 → prophylactic proctocolectomy; screen relatives at 10–12 yr; periampullary cancer = top cause of death post-colectomy. Gardner = FAP + desmoids/osteomas; Turcot = FAP/Lynch + brain tumors. Lynch (HNPCC) = AD mismatch-repair (MLH1/MSH2), right-sided, Amsterdam 3-2-1-1, also endometrial/ovarian/gastric → total colectomy + ileorectal anastomosis.
• Sigmoid volvulus — elderly/institutionalized, "coffee-bean/bent inner tube" to RUQ → endoscopic detorsion (nontoxic), then semi-elective sigmoid colectomy (high recurrence). Cecal volvulus — younger, coffee bean to LUQ → NO scope; ileocecectomy/right hemicolectomy (gangrene).
• Ulcerative colitis — continuous, rectum→proximal, mucosa/submucosa only, crypt abscesses, pseudopolyps, "lead-pipe" colon. Medical: 5-ASA (sulfasalazine/mesalamine) → steroids for flares → infliximab/thiopurines. Cancer surveillance colonoscopy 8 yr after dx, then annually; flat dysplasia. Curative = total proctocolectomy with ileal pouch–anal anastomosis (IPAA). Most common late IPAA complication = pouchitis (Flagyl/cipro).
• Toxic megacolon — colitis + distention/systemic toxicity → NG decompression, IV fluids, bowel rest, antibiotics; ~50% fail → subtotal colectomy + end ileostomy. UC perforation site = transverse colon; Crohn = distal ileum.
• Diverticulosis = false (pulsion) diverticula at vasa recta penetration points, #1 cause of lower GI bleed (right-sided, arterial, 75% stop spontaneously). Diverticulitis = LLQ pain/fever/leukocytosis → CT; uncomplicated → bowel rest + antibiotics; colonoscopy in ~6 wk to exclude cancer. Abscess (most common complication) → percutaneous drainage; feculent/purulent peritonitis (Hinchey III–IV) → Hartmann procedure.
• Lower GI bleed workup: ABCs → NGT (rule out upper source) → colonoscopy if stable; if ongoing/unlocalized → CTA then angiography (embolization) or tagged-RBC scan; unstable + unlocalized → total colectomy.
• Ischemic colitis — low-flow at watershed areas, elderly + CV disease; pain + bloody diarrhea, colonoscopy is best diagnostic (segmental); usually medical (bowel rest, antibiotics), surgery for peritonitis/deterioration.
• C. difficile (pseudomembranous) colitis — post-antibiotics, yellow plaques; treat with oral vancomycin or fidaxomicin (NOT IV vanc; metronidazole is 2nd-line). Fulminant → PO/PR vanc + IV metronidazole; toxic megacolon → subtotal colectomy.
• Ogilvie syndrome (colonic pseudo-obstruction) — bedridden/elderly, no mechanical block. Perforation risk when cecum >12 cm → exclude distal obstruction → conservative; neostigmine (cardiac monitor, atropine ready) → endoscopic decompression → cecostomy.

Key Tables / Differentials

Board Pearls

• Strep bovis (gallolyticus) endocarditis → colonoscopy to find colorectal cancer.
• Apple-core lesion on barium / iron-deficiency anemia in an older adult → left-sided CRC.
• Pneumaturia/fecaluria + recurrent UTIs → colovesical fistula (diverticulitis #1) → CT.
• Most common site of colonic perforation = cecum (Laplace); obstruction with competent ileocecal valve = closed loop.
• Diversion colitis → short-chain fatty acid enemas.
• Right-sided diverticulitis → medical management (NPO, IV fluids, antibiotics).

Classic Vignette Triggers

• "Thousands of polyps, AD, APC mutation" → FAP → prophylactic proctocolectomy.
• "Right-sided cancer, AD, endometrial cancer, Amsterdam criteria" → Lynch.
• "Bloody diarrhea, continuous from rectum, crypt abscesses" → UC.
• "Transmural skip lesions, terminal ileum, granulomas, fistula" → Crohn.
• "Coffee-bean loop pointing to the RUQ" → sigmoid volvulus → scope detorsion.
• "Profuse diarrhea after antibiotics, yellow plaques" → C. diff → oral vancomycin/fidaxomicin.
• "Bedridden patient, massive colonic distention, no mechanical obstruction, cecum >12 cm" → Ogilvie → neostigmine.
• "Painless massive hematochezia, elderly, stops on its own" → diverticular bleed.
• "Postprandial pain + bloody diarrhea at the splenic flexure" → ischemic colitis.