What are the "Rule of 2's" for Meckel diverticulum?
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Answer
2% of population, 2 feet from ileocecal valve, 2 inches long, 2 tissue types (gastric + pancreatic), symptomatic by age 2, 2:1 male:female. Caused by failure of vitelline duct regression. Tc-99m pertechnetate scan detects ectopic gastric mucosa.
LO 1 β Meckel Complications
What is the most common symptomatic complication of Meckel in children?
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Answer
Painless rectal bleeding (hemochezia) due to ectopic gastric mucosa secreting acid β ulceration of adjacent ileum. Most common cause of painless lower GI bleed in children under 2 years. Other complications: obstruction, volvulus, intussusception, diverticulitis.
LO 1 β Hirschsprung Pathogenesis
What genetic defect causes Hirschsprung disease and what is the result?
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Answer
Loss-of-function RET mutation β failure of neural crest cell migration β absence of ganglion cells in myenteric/submucosal plexuses β aganglionic rectosigmoid segment β no peristalsis β functional obstruction. Associated with Down syndrome (trisomy 21).
LO 1 β Hirschsprung Diagnosis
What is the classic presentation and diagnostic finding in Hirschsprung disease?
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Answer
Presentation: failure to pass meconium within 48 hours + abdominal distension in newborn. Diagnosis: rectal biopsy showing absence of ganglion cells (histologic gold standard). The aganglionic zone is grossly normal; the dilated megacolon is the normal innervated segment proximal to blockage.
LO 2 β Intussusception Lead Points
How does the lead point differ between intussusception in children vs. adults?
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Answer
Children: most common cause of SBO in infants/young children; often idiopathic or from Peyer's patch hypertrophy after viral illness. Adults: always suspect a mass (polyp, tumor, lipoma) as lead point β higher malignancy concern. Location: ileocecal junction most common (SI telescopes into colon).
LO 2 β Volvulus Classic Sign
What is the classic imaging finding for sigmoid volvulus and in what population is it most common?
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Answer
Classic sign: "coffee bean" appearance on plain radiograph. Demographics: elderly patients (most common site = sigmoid colon due to loose mesenteric attachment); cecal volvulus occurs in younger patients. Causes luminal AND vascular compromise β obstruction + infarction risk if prolonged.
LO 2 β Adhesions #1 Cause SBO
What is the #1 cause of small bowel obstruction and what conditions cause adhesions?
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Answer
Adhesions = #1 cause of SBO. Caused by prior abdominal surgery (most common), but also peritonitis, endometriosis, IBD, appendicitis, and any inflammatory peritoneal process. Fibrous bands restrict intestinal mobility β kinking β obstruction β ischemia risk.
LO 3 β Ischemic Colitis Watershed
What are the two watershed zones vulnerable to ischemic injury and what vascular boundaries define them?
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Answer
Splenic flexure: border between SMA (right colon) and IMA (left colon). Rectosigmoid: border between IMA and internal iliac artery. These zones have limited collateral circulation β highest risk for ischemia. Imaging shows "thumbprinting" (edematous mucosa + submucosal hemorrhage).
LO 3 β Mesenteric Ischemia Red Flags
What are the classic symptoms of acute mesenteric ischemia and why is it high-mortality?
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Answer
Classic triad: severe abdominal pain out of proportion to exam findings + bloody diarrhea (late sign) + signs of shock. High mortality: because pain is out of proportion (ischemic bowel fails early; pain signals severe tissue damage). Risk factors: AF (thromboembolism), atherosclerosis (thrombosis), vasculitis, cocaine use (vasoconstriction).
LO 4 β Celiac Disease HLA Types
What HLA types are associated with celiac disease and what are the serologic tests?
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Answer
HLA-DQ2 (95%) or HLA-DQ8 (5%) present deamidated gliadin peptides β immune response. Serology: anti-tTG (tissue transglutaminase) IgA, anti-endomysial antibodies (highly specific). Note: IgA deficiency (~2-3% of patients) can cause false-negative serology β test for total IgA first.
LO 4 β Celiac Duodenal Biopsy
What are the pathologic findings on duodenal biopsy in celiac disease?
What is the major malignancy risk in untreated celiac disease?
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Answer
Enteropathy-associated T-cell lymphoma (EATL): intraepithelial T-cell lymphoma that arises from chronic gluten-induced intestinal damage. Gluten-free diet reduces but does not eliminate risk. Also at risk: dermatitis herpetiformis (gluten-responsive blistering dermatosis), iron/folate/B12 deficiency, osteoporosis (malabsorption).
LO 5 β Crohn vs. UC Buzzwords
What are the classic histologic/imaging buzzwords that distinguish Crohn from UC?
Which IBD has higher cancer risk and which is associated with PSC?
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Answer
Both increase CRC risk (continuous mucosal inflammation β dysplasia). UC: strongly associated with primary sclerosing cholangitis (PSC) β 10-fold increased cholangiocarcinoma risk. PSC screening = ERCP/MRCP; PSC + UC requires colonoscopy surveillance. Toxic megacolon can occur in both but classically UC.
LO 6 β Diverticulosis False Diverticula
Are colonic diverticula true or false diverticula and why?
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Answer
False (pseudo) diverticula: only mucosa + submucosa herniate through muscular wall at sites where vasa recta penetrate muscularis. Sigmoid colon most common due to taeniae coli (muscle bands). Risk factors: low-fiber diet, increased intraluminal pressure, aging. Painless hematochezia is common presenting symptom.
LO 6 β Diverticulitis Complications
What are the major complications of diverticulitis and how do they present?
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Answer
Uncomplicated: LLQ pain, fever, leukocytosis. Complicated: abscess (may be percutaneously drained), perforation (free air, peritonitis), fistula (colovesical = dysuria + pneumaturia), obstruction. CT = imaging gold standard. Recurrent diverticulitis after first attack increases risk of complications.
LO 7 β Adenoma Malignant Potential
Which adenoma subtype has highest malignant potential and why?
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Answer
Villous adenoma (40-50% cancer risk) > tubulovillous (10-25%) > tubular (5-10%). Villous has longest villous fronds β greater mucosal surface area β higher dysplasia rate. Sessile serrated polyps (flat appearance) also have high cancer risk. Hyperplastic polyps (benign, rectosigmoid) do NOT progress to cancer.
LO 7 β Hamartomatous Syndromes
What are the key features of Peutz-Jeghers and Juvenile Polyposis syndromes?
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Answer
Peutz-Jeghers: STK11 mutation, multiple hamartomas throughout GI tract, mucocutaneous pigmentation (dark lips/buccal mucosa), 30-50% cancer risk (GI + extra-GI). Juvenile polyposis: SMAD4/BMPR1A mutations, juvenile polyps in colon/rectum, high bleeding/intussusception risk, 20% CRC risk. Both inherited autosomal dominant.
LO 8 β FAP Genetics & Cancer
What genetic defect causes FAP and what is the cancer risk?
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Answer
APC gene mutation (chr 5q21) β loss of Ξ²-catenin regulation β unopposed Wnt signaling β hundreds/thousands of colonic adenomas. 100% cancer risk by age 40 without prophylactic colectomy. Gardner variant: + osteomas, desmoid tumors, dental abnormalities. Turcot variant: + CNS tumors (medulloblastoma, glioma).
LO 8 β Lynch Syndrome (HNPCC)
How does Lynch syndrome differ from FAP in genetics and cancer pattern?
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Answer
Lynch (HNPCC): MSH2/MLH1/MSH6/PMS2 mismatch repair gene mutations β microsatellite instability (MSI) β right-sided, flat adenomas (harder to detect) β 80% CRC risk by age 70. Associated with endometrial cancer (more common than CRC in women). APC wild-type (FEW polyps, but HIGH cancer risk). Screening: annual colonoscopy from age 25.
LO 8 β CRC Presentation & Anatomy
How does CRC presentation differ between left-sided vs. right-sided tumors?
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Answer
Left-sided (sigmoid/rectum): annular "napkin-ring" appearance, obstruction symptoms, bright red blood per rectum, constipation. Right-sided (cecum/ascending): exophytic mass, iron deficiency anemia (chronic bleeding), palpable mass, less obstruction (wider lumen). Adenoma-carcinoma sequence: APC loss β KRAS activation β TP53 loss β metastasis (~10-15 years).
LO 2 β Volvulus Pathophysiology
Order the pathophysiologic sequence of sigmoid volvulus complications.
Bowel twists around mesentery
Luminal obstruction develops
Mesenteric vessels compressed
Bowel wall ischemia β infarction
Perforation and peritonitis
Bowel twists around mesentery β the mechanical twist
Peritonitis and septic shock β generalized peritoneal infection; requires emergency surgery
MCQ 1
A 2-year-old boy presents with painless rectal bleeding and a hemoglobin of 9.2 g/dL. Upper and lower endoscopy are normal. A technetium-99m pertechnetate scan reveals a small area of uptake in the terminal ileum. What is the most likely diagnosis?
AJuvenile polyp
BIntussusception
CMeckel diverticulum
DInflammatory bowel disease
EColonic adenoma
Answer: C β Meckel diverticulum. Tc-99m pertechnetate (sodium pertechnetate) localizes to ectopic gastric mucosa (which produces gastrin-like peptides), making this the gold standard imaging for Meckel. The "rule of 2's" (2%, 2 feet from IC valve, 2 inches long, 2 tissues, symptomatic by age 2) fits. Painless GI bleed in children <2 yrs = Meckel until proven otherwise. Distractors: juvenile polyps bleed but are benign and don't light up on Tc-99m; intussusception causes pain; IBD causes pain + diarrhea; adenomas rare in young children.
MCQ 2
A 78-year-old woman with chronic constipation presents with acute abdominal distension, severe pain, and a characteristic "coffee bean" sign on abdominal radiograph. CT shows twisted colon with compromised blood supply. What is the immediate management?
AHigh-fiber diet and stool softeners
BUrgent endoscopic decompression; surgical resection if gangrenous
CNPO and observation for 48 hours
DColonoscopy with biopsy
ENasogastric tube decompression only
Answer: B β Endoscopic decompression is first-line for uncomplicated sigmoid volvulus. The "coffee bean" sign + elderly patient = classic sigmoid volvulus. If endoscopy succeeds, most cases resolve; if it fails or bowel is gangrenous, emergency surgical resection is needed (cannot be delayed). A diet change is too slow. Observation risks necrosis/perforation. Biopsy doesn't decompress. NG tube alone doesn't relieve the twist.
MCQ 3
A 35-year-old man with celiac disease (confirmed by duodenal biopsy showing villous atrophy) is counseled to start a gluten-free diet. Despite excellent adherence after 2 years, repeat biopsy shows persistent villous atrophy and crypt hyperplasia. What complication should you screen for most aggressively?
ASecondary lactose intolerance
BIrritable bowel syndrome
CPrimary biliary cholangitis
DUlcerative colitis
EEnteropathy-associated T-cell lymphoma (EATL)
Answer: E β Refractory celiac disease (persistent villous atrophy despite strict gluten-free diet) is a major risk factor for EATL (intraepithelial T-cell lymphoma). This is a life-threatening complication. The presence of aberrant T-cell clones in the epithelium signals high malignancy risk. Regular endoscopic surveillance is needed. Distractors: secondary lactose intolerance is common in active celiac but usually resolves with diet; IBS/PSC/UC can coexist but are not specifically associated with refractory disease.
MCQ 4
A 45-year-old woman with known Crohn disease (terminal ileum) presents with fistulous drainage near her rectum. Imaging shows a fistula tract connecting the colon to the skin. Which pathologic feature best explains fistula formation in Crohn disease?
ASuperficial ulceration limited to mucosa and submucosa
BCrypt abscesses with pseudopolyp formation
CTransmural inflammation with deep ulcerations penetrating through all bowel layers
DIntestinal strictures from smooth muscle hypertrophy
EAdenomatous polyp degeneration
Answer: C β Crohn disease is transmural (involves all layers) β deep ulcerations can penetrate through the full bowel wall β fistula formation to adjacent bowel, bladder, or skin. This is a hallmark of Crohn, distinguishing it from UC (which is mucosal/submucosal only). UC β crypt abscesses, not fistulas. Strictures occur but don't explain fistula formation. Adenomas are not part of Crohn pathology.
MCQ 5
A 62-year-old man with a 40-pack-year smoking history presents with acute left lower abdominal pain, fever (38.8Β°C), and leukocytosis (WBC 14,500). CT shows a localized abscess (4 cm) adjacent to the sigmoid colon with no free air. What is the most appropriate next step?
APercutaneous drain placement under imaging guidance
BEmergency surgical colectomy
CBroad-spectrum antibiotics and observation
DColonoscopy with stricture dilation
EElective colectomy scheduled for 3 months
Answer: A β Diverticulitis with localized abscess (no free perforation) is best managed with percutaneous drain + antibiotics. This percutaneous approach (CT or ultrasound-guided) allows infection control without emergency surgery. B (surgery) is for free perforation or failed drainage. C (antibiotics alone) may work for very small abscesses <4 cm, but larger ones need drainage. D (colonoscopy) is not indicated during acute inflammation. E is for recurrent diverticulitis or obstructing disease, not acute abscess.
MCQ 6
A 68-year-old man with no significant past medical history presents with iron deficiency anemia (Hgb 8.5 g/dL) and occult fecal blood. Colonoscopy reveals a large, exophytic, fungating mass in the cecum. What is the classic presentation pattern for right-sided colorectal cancer?
AAcute obstruction with abdominal pain and constipation
BTenesmus and bright red blood per rectum with change in bowel habits
CIron deficiency anemia from chronic occult bleeding with exophytic mass
DAcute perforation and peritonitis
EFecal impaction and palpable stool in abdomen
Answer: C β Right-sided (cecal/ascending) CRC presents late with anemia from chronic occult bleeding. The wider lumen delays obstruction, so patients bleed slowly β iron deficiency. Mass is typically exophytic/fungating (bleeds easily). Left-sided (sigmoid) presents with obstruction (annular "napkin-ring" lesion, narrower lumen) and bright red blood. Perforation occurs but is less common than in left-sided. Tenesmus (feeling of incomplete evacuation) = left-sided/rectal sign.
MCQ 7
A 55-year-old woman is found to carry a pathogenic MLH1 mutation on genetic testing. She has a history of endometrial cancer (diagnosed at age 38) and her brother had colorectal cancer at age 45. What malignancy screening protocol should she follow?
AAnnual colonoscopy starting at age 50; gynecologic exam every 5 years
BProphylactic total proctocolectomy and hysterectomy
CFlexible sigmoidoscopy every 3 years; PAP smear annually
DAnnual colonoscopy starting at age 25; annual gynecologic ultrasound/biopsy starting at age 30
EGenetic counseling only; no screening if no symptoms
Answer: D β Lynch syndrome (HNPCC with MLH1 mutation) mandates aggressive surveillance: annual colonoscopy from age 25 (or 5 years before youngest relative's CRC diagnosis) + annual gynecologic screening (endometrial cancer is MOST common Lynch-associated cancer in women). Her early endometrial cancer + brother's early CRC = classic Lynch presentation. A (age 50) is too late. B (prophylactic surgery) is not standard unless prior cancer. C (sigmoidoscopy) misses right-sided adenomas. E ignores high cancer risk.
MCQ 8
A 40-year-old man develops sudden severe abdominal pain and bloody diarrhea. He has a history of atrial fibrillation (not on anticoagulation). CT angiography shows SMA thrombosis with intestinal wall thickening and thumbprinting. What is the most critical feature of acute mesenteric ischemia that explains the delayed diagnosis?
APain is out of proportion to physical exam findings (early sign)
BBloody diarrhea is the earliest and most specific sign
CFever is always present, helping distinguish from other causes
DLeucocytosis with left shift is pathognomonic
EDistension and constipation precede bloody diarrhea by weeks
Answer: A β The hallmark and most dangerous feature of acute mesenteric ischemia is pain out of proportion to exam. The ischemic mucosa is dying (severe pain signal) but the abdomen may initially appear soft/non-peritoneal. By the time peritonitis develops, transmural infarction is likely β 50%+ mortality. Bloody diarrhea is a LATE sign (mucosa breaking down). Risk factors: AF β thromboembolism, atherosclerosis β thrombosis. High clinical suspicion is critical.
MCQ 10
A 52-year-old woman with known familial adenomatous polyposis (APC mutation) undergoes surveillance colonoscopy and is found to have hundreds of colonic adenomas and three adenomas with high-grade dysplasia. She is counseled regarding prophylactic colectomy. What is the rationale for this recommendation?
ATo prevent spontaneous bowel perforation from polyp overgrowth
BTo prevent almost-certain colorectal cancer development (100% risk by age 40β50 without surgery)
CTo improve fertility and reduce genetic transmission risk
DTo prevent adenoma regrowth after endoscopic polypectomy
ETo reduce the need for lifelong colonoscopic surveillance
Answer: B β FAP has 100% colorectal cancer risk by age 40β50 if untreated (due to unopposed Wnt signaling from APC loss). Prophylactic colectomy (total proctocolectomy with ileoanal pouch anastomosis or permanent ileostomy) is standard of care once adenomas with high-grade dysplasia appear. A (perforation) is not the main driver. C (fertility) is not indication. D (regrowth) is managed by surveillance. E (surveillance) is still needed even after colectomy because adenomas can recur in residual rectum/pouch.
Vignette 1
Case: A 6-month-old male infant presents with intermittent bouts of colicky abdominal pain, irritability, and three episodes of currant-jelly stools (mucus mixed with blood). On abdominal exam, a palpable "sausage-shaped" mass is felt in the RUQ. Ultrasound confirms a "donut sign" and "target sign" in the right abdomen. What is the diagnosis and most appropriate management?
AIntussusception; air or contrast enema for both diagnostic confirmation and therapeutic reduction
BHirschsprung disease; rectal suction biopsy
CMeckel diverticulum; technetium-99m scan
DInfectious colitis; stool studies and antibiotics
Answer: A β Classic intussusception: currant-jelly stools + palpable sausage mass + ultrasound "donut" and "target" signs. Most common cause of bowel obstruction in infants 6β36 months. Ileocecal junction is the typical site (SI telescopes into colon due to difference in peristaltic waves). Therapeutic reduction via air or contrast enema (hydrostatic or pneumatic reduction) is first-line if no peritoneal signs/perforation. Success rate ~80β90% in uncomplicated cases. Requires surgical reduction if enema fails. Distractors: Hirschsprung presents with failure to pass meconium in neonatal period (not colicky pain or blood); Meckel bleeding is painless; infectious/IBD are less acute.
Vignette 2
Case: A 56-year-old woman with a 20-year history of ulcerative colitis (total colon involvement) presents with RUQ pruritus, pale stools, and dark urine. Liver biochemistry shows ALP 320 U/L, ALT 45 U/L, total bilirubin 2.8 mg/dL. MRCP shows a beaded appearance of the intrahepatic ducts with stricturing. What is the most likely diagnosis and its management implication?
APrimary sclerosing cholangitis (PSC); requires ERCP surveillance and endometrial cancer screening
Answer: A β UC is strongly associated with primary sclerosing cholangitis (PSC), a chronic cholangiopathy causing intrahepatic and extrahepatic bile duct stricturing. MRCP shows "beaded" ducts (strictures + dilation). Management: ERCP surveillance (can be therapeutic for dominant strictures), monitoring for cholangiocarcinoma. Critical: endometrial cancer screening because women with UC + PSC have 10-fold increased cholangiocarcinoma and endometrial cancer risk. Ursodeoxycholic acid may be used but doesn't alter PSC progression. AIH/PBC are autoimmune conditions with different serologies and MRCP patterns. Cholecystitis/lithiasis are different pathology.
Vignette 3
Case: A 32-year-old man with known Crohn disease (terminal ileum) on remission-induction therapy presents with sudden onset of severe RLQ pain, fever (39.1Β°C), and a palpable abdominal mass. CT shows a thick-walled terminal ileal segment with dilated proximal small bowel and a 6 cm complex fluid collection. What is the most likely complication and management?
AIntussusception; contrast enema for reduction
BToxic megacolon; broad-spectrum antibiotics and surgical consult
DEnterocutaneous fistula; supportive care and nutritional support
EDysplasia/early malignancy; surgical resection of involved bowel
Answer: C β Crohn disease complicated by intra-abdominal (mesenteric) abscess: transmural inflammation β fistulization or perforation β localized abscess. Classic presentation: fever, RLQ pain, palpable mass (abscess), elevated inflammatory markers. Management: percutaneous drainage (CT/ultrasound-guided) + broad-spectrum antibiotics (covering gram-negative and anaerobes). If drainage fails, fever persists, or recurrent abscess β surgical resection of diseased segment. Intussusception in Crohn is rare. Toxic megacolon is UC-classic (not Crohn). Fistulas are chronic complications. Dysplasia/cancer is rare in Crohn despite increased CRC risk.
Vignette 4
Case: A 68-year-old man with no prior medical history presents with acute onset severe abdominal pain (pain 10/10, disproportionate to exam) and bloody stools. He has a history of atrial fibrillation (not anticoagulated), hyperlipidemia, and recent MI. Vitals show tachycardia (HR 110) and hypotension (BP 88/52). Abdominal exam is surprisingly soft with minimal tenderness. CT angiography shows SMA thrombosis. What is the pathophysiology of this presentation?
AHemorrhagic colitis with intact blood supply; inflammatory cytokine release
BAcute transmural bowel necrosis from ischemia; mucosal breakdown and bacterial translocation β endotoxemia + shock; pain reflects ischemic tissue death before peritoneal signs develop
CLocalized perforation with chemical peritonitis; limited to affected segment
DToxic megacolon with profound ileus; pain from bowel wall dilation
EObstructive ileus from intraluminal thrombus; pain from upstream bowel dilation
Answer: B β Acute mesenteric ischemia is a surgical emergency.SMA thrombosis (often from AFib, atherosclerosis, or hypercoagulable state) stops blood flow β mucosal ischemia (earliest manifestation) β full-thickness necrosis within hours β gram-negative bacteria + endotoxin translocate β septic shock. Hallmark: pain out of proportion to exam. The soft abdomen is because peritonitis hasn't developed yet (tissue is dying, but free perforation hasn't occurred). By the time peritoneal signs appear, transmural necrosis is likely established β 50% mortality even with surgery. Risk factors: AFib (most common cause of mesenteric embolism), atherosclerosis, vasculitis. Mortality rises with delay; urgent surgical revascularization/resection required.
Vignette 5
Case: A 48-year-old woman with a family history of colorectal cancer (mother diagnosed at age 52, father at age 55) and endometrial cancer (aunt at age 42) undergoes genetic testing and is found to carry a pathogenic MSH2 mutation. She undergoes colonoscopy and several flat, nonpolypoid adenomas are found in the right colon. What is the genetic mechanism explaining her phenotype?
AAPC mutation β hundreds of adenomas; adenoma-carcinoma sequence over 10β15 years
BLynch syndrome (HNPCC); mismatch repair gene loss (MSH2) β microsatellite instability (MSI) β right-sided flat adenomas + high CRC/endometrial cancer risk
CPTEN mutation β Cowden syndrome with hamartomas and breast/thyroid cancer risk
DSTK11 mutation β Peutz-Jeghers syndrome with mucocutaneous pigmentation and GI polyps
ESporadic colorectal cancer with poor prognosis; no familial/genetic basis
Answer: B β Lynch syndrome (HNPCC): loss-of-function mutation in mismatch repair genes (MSH2, MLH1, MSH6, PMS2) β microsatellite instability (MSI-High) β impaired DNA repair β rapid accumulation of mutations. Phenotype: flat, right-sided adenomas (harder to detect!) + early-onset CRC + endometrial cancer.Cancer risk: 80% CRC, 40% endometrial cancer by age 70. Screening: annual colonoscopy from age 25 (or 5 years before youngest relative's diagnosis) + annual gynecologic ultrasound/biopsy. FAP (A) presents with hundreds of polyps. Cowden/Peutz-Jeghers (C/D) have different genetic/clinical profiles. This family history + MSH2 mutation + flat right-sided adenomas = classic Lynch.
Trap 1
Diaphragmatic hernia (congenital) is a common cause of intestinal obstruction in neonates.
Correct answer: FALSE. Congenital diaphragmatic hernias cause respiratory distress (compressed lungs) in neonates, NOT intestinal obstruction. They present acutely after birth with dyspnea, not bowel obstruction. The most common causes of neonatal SBO are: meconium ileus (CF), intestinal atresia, malrotation (volvulus), and Hirschsprung disease. Trap: students confuse "hernia" with "bowel obstruction," but diaphragmatic hernias are primarily pulmonary emergencies.
Trap 2
Ulcerative colitis is NOT associated with primary sclerosing cholangitis (PSC); only Crohn disease has this association.
Correct answer: FALSE.UC is STRONGLY associated with PSC (much more than Crohn). PSC occurs in ~5β10% of UC patients and is the #1 indication for liver transplantation in UC-related liver disease. Why the trap? Students learn "UC = PSC; Crohn = bile duct symptoms," but both IBDs increase cholangiocarcinoma risk; PSC is more common in UC. Crohn can have perianal disease and fistulas (different GI complications) but PSC is not a Crohn hallmark.
Trap 3
The most common location of a true diverticulum (Meckel) is the jejunum.
Correct answer: FALSE.Meckel diverticulum is always in the TERMINAL ILEUM, within 2 feet of the ileocecal valve. It arises from incomplete regression of the vitelline (omphalomesenteric) duct, which is embryologically associated with the distal small bowel. Why the trap? Students sometimes confuse "small intestine diverticula" broadly, but Meckel's classic location is terminal ileum only. Jejunal true diverticula exist but are rare and not on Step 1.
Trap 4
Celiac disease is caused by a direct toxic effect of gluten on intestinal epithelial cells.
Correct answer: FALSE. Celiac is immune-mediated, not direct toxicity. Gluten peptides are deamidated by tTG β HLA-DQ2/DQ8 presents them β T-cell response β CD8+ intraepithelial lymphocytes infiltrate epithelium β apoptosis of enterocytes β villous atrophy. Why the trap? The term "gluten-sensitive" makes it sound chemical/toxic, but it's fundamentally a T-cell mediated autoimmune process that requires both HLA susceptibility AND gluten. Genetic studies prove it: HLA-DQ2/DQ8 are essential. Non-celiac gluten sensitivity (NCGS) exists but is much less common and not immune-mediated in the same way.
Trap 5
Fistula formation is common in both Crohn disease and ulcerative colitis at equal rates.
Correct answer: FALSE.Fistulas are a hallmark of CROHN DISEASE, NOT UC. Crohn is transmural (all layers) β deep ulcers penetrate bowel wall β fistula to adjacent bowel (enteroenteric), bladder (colovesical), skin (enterocutaneous), or abscess. UC is mucosa/submucosa only β crypt abscesses and pseudopolyps, but NO fistulas. Why the trap? Students sometimes lump Crohn and UC together ("IBD complications"), but the transmural nature of Crohn is the key differentiator. Fistulas = Crohn; crypt abscesses/toxic megacolon/PSC = UC.
Trap 6
Patients with hereditary adenomatous polyposis (FAP) develop a few hundred polyps, managed with frequent colonoscopy and polypectomy.
Correct answer: FALSE. FAP patients develop hundreds to THOUSANDS of adenomatous polyps β 100% cancer risk by age 40β50 β prophylactic total colectomy is standard of care (not surveillance + polypectomy). Why the trap? "Frequent colonoscopy" works for Lynch syndrome (MSI pathway: few adenomas, high cancer risk) and sporadic adenomas, but NOT for FAP. The sheer number and inevitable malignant transformation make surgery essential. Students may confuse the management of FAP vs. Lynch: FAP = surgery, Lynch = surveillance from young age.
Trap 7
Watershed ischemia in the colon is most commonly caused by low blood pressure in diabetics or during dehydration.
Correct answer: FALSE (partially). Watershed ischemia occurs due to limited COLLATERAL CIRCULATION at boundaries between major arteries (splenic flexure = SMA/IMA; rectosigmoid = IMA/internal iliac), NOT simply "low blood pressure." While hypotension can precipitate ischemia, the watershed zones are structurally vulnerable because of anatomy, not metabolic factors. Why the trap? Students sometimes think ischemic colitis = "not enough blood overall," but it's specifically about vascular territory anatomy. Even with normal systemic BP, occlusion of the IMA or SMA can cause watershed ischemia if collaterals are inadequate.
Trap 8
Hirschsprung disease affects the entire colon and always causes meconium-stained amniotic fluid in utero.
Correct answer: FALSE. Hirschsprung affects the rectosigmoid ALWAYS (extends proximally variable lengths, but rectum is always aganglionic). The rectum is always involved. Meconium staining is NOT a prenatal finding because Hirschsprung presents after birth with failure to pass meconium (within 48 hrs). Why the trap? Students confuse "Hirschsprung involves gut" with "affects entire colon," but aganglionosis always starts at the internal anal sphincter (rectum) and extends proximally. Meconium staining is a sign of fetal distress in utero, not a prenatal marker of Hirschsprung.