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GI Pathology — Step 1 High-Yield Review

(From First Aid p. 394: Zenker Diverticulum, Meckel Diverticulum, Hirschsprung Disease)

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1. ZENKER DIVERTICULUM

What It Is

Anatomy — The Key

• Thyropharyngeus muscle (oblique fibers, upper)
• Cricopharyngeus muscle (horizontal fibers, lower)

Who Gets It

• Predominantly elderly males
• Associated with hiatal hernia and GERD

Classic Symptoms (Remember ALL — commonly tested)

Diagnosis

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🔴 Step 1 MCQ High-Yield Points — Zenker

1. False diverticulum = only mucosa/submucosa herniate (vs. Meckel = TRUE diverticulum)
2. Location: Posterior hypopharynx, between thyropharyngeus and cricopharyngeus at Killian's triangle
3. Caused by esophageal dysmotility (NOT just aging)
4. Classic triad: dysphagia + halitosis + regurgitation of undigested food
5. Most common in elderly males
6. Diagnosis: Barium swallow (NOT endoscopy first — risk of perforation)
7. Complication: aspiration pneumonia
8. Treatment: Surgical myotomy (cutting the cricopharyngeus)
9. Distinguish from mid-esophageal (traction, true) and epiphrenic diverticula — Zenker is PHARYNGOESOPHAGEAL

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2. MECKEL DIVERTICULUM

What It Is

Location

Ectopic Tissue

• 80–85% is gastric mucosa → secretes acid → causes peptic ulceration at the gastric-ileal mucosal junction → painless rectal bleeding (most common presentation in children)
• Also: pancreatic tissue, duodenal, colonic tissue

The Rule of 2's (MOST TESTED FACT)

Clinical Presentations

Diagnosis

Treatment

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🔴 Step 1 MCQ High-Yield Points — Meckel

1. True diverticulum (all 3 layers) — most common congenital anomaly of the GI tract
2. Rule of 2's — memorize all 6 twos
3. Ectopic gastric mucosa is responsible for bleeding (acid production)
4. Painless maroon/melena rectal bleeding in a child → think Meckel's
5. Diagnosis = ⁹⁹ᵐTc-pertechnetate (Meckel) scan
6. Can mimic appendicitis (RLQ pain, Meckel diverticulitis)
7. Can act as lead point for intussusception in children >4 years
8. Persistence of vitelline (omphalomesenteric) duct
9. Anti-mesenteric border → distinguishes from appendix (mesenteric)

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3. HIRSCHSPRUNG DISEASE

What It Is

Pathogenesis (KEY)

• Neural crest cells normally migrate craniocaudally into the bowel wall during embryogenesis
• In Hirschsprung, this migration arrests prematurely — cells fail to reach the distal bowel
• Result: absent Meissner's submucosal plexus AND Auerbach's (myenteric) plexus in the affected segment
• Without these plexuses → no coordinated peristalsis → functional obstruction
• The aganglionic segment is tonically contracted and collapsed (no relaxation)
• The proximal, normally innervated colon dilates (megacolon) from the buildup

Genetics

• RET proto-oncogene loss-of-function mutations — majority of familial cases (~50%) and ~15% of sporadic cases
• RET = RECeptor tyrosine kinase — mnemonic: RET mutation in the RECTum
• Associated with Down syndrome (trisomy 21) — increased risk (exact mechanism unclear)
• More common in males, but more severe in females

Presentation

Transition Zone

Diagnosis

Treatment

Complications

• Hirschsprung-associated enterocolitis (HAEC) — life-threatening; from bacterial overgrowth proximal to obstruction
• Fluid/electrolyte disturbances, perforation, peritonitis

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🔴 Step 1 MCQ High-Yield Points — Hirschsprung

1. Aganglionic distal segment = absent Meissner's AND Auerbach's plexuses
2. Caused by failure of neural crest cell migration (not degeneration)
3. RET mutation (loss-of-function) — familial and some sporadic cases
4. Associated with Down syndrome (trisomy 21)
5. More common in males, but more severe in females
6. Neonate: failure to pass meconium >48 hours + abdominal distention
7. "Squirt sign" on digital rectal exam → empty rectum (narrow aganglionic rectum)
8. Diagnosis: rectal suction biopsy showing absence of ganglion cells
9. Transition zone on barium enema = diagnostic radiological finding
10. Aganglionic segment is collapsed/narrow; proximal colon is dilated
11. Treatment: surgical resection of aganglionic segment

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Quick Comparison Table

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GI Pathology — Step 1 High-Yield Review

(First Aid p. 395: Malrotation & Intussusception)

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1. MALROTATION

What It Is

Normal Embryology (You Must Know This)

• At week 6, the midgut grows too fast for the abdomen → herniates into the umbilical cord
• Between weeks 10–12, it returns to the abdomen undergoing a 270° counterclockwise rotation around the superior mesenteric artery (SMA)
• The duodenum rotates posterior to the SMA → forms the C-loop (retroperitoneal, fixed at ligament of Treitz)
• The cecum rotates to the right lower quadrant and gets fixed to the lateral abdominal wall

What Goes Wrong in Malrotation

• Rotation is incomplete → cecum stays in the epigastrium (instead of RLQ)
• Peritoneal bands still form from cecum → lateral abdominal wall = Ladd bands
• Ladd bands cross over the duodenum → cause duodenal obstruction
• The mesenteric takeoff remains narrow (confined to epigastrium) → entire midgut hangs on a short, narrow pedicle
• This narrow pedicle predisposes to midgut volvulus — twisting cuts off the SMA → bowel ischemia/infarction if not corrected

Key Anatomical Result

• Small bowel clumped on the RIGHT side of the abdomen (abnormal)
• Colon on the LEFT (abnormal)
• Ladd bands compress the second portion of the duodenum

Clinical Presentation

Diagnosis

• Upper GI series (barium) — best initial test; shows:
  • Duodenojejunal junction displaced to the RIGHT (normally left of midline at ligament of Treitz)
  • "Corkscrew" appearance of duodenum/jejunum = volvulus
  • Small bowel loops entirely on the right side
• Abdominal X-ray: "double bubble" sign (if duodenal obstruction), paucity of distal bowel gas
• Barium enema: displaced cecum (less reliable)

Treatment

1. Detorse the volvulus (counterclockwise)
2. Divide Ladd bands
3. Broaden the mesenteric base
4. Appendectomy (prophylactic, since appendix ends up on the left)

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🔴 Step 1 MCQ High-Yield Points — Malrotation

1. Midgut normally rotates 270° counterclockwise around the SMA between weeks 10–12
2. Malrotation = incomplete rotation → cecum stays in epigastrium
3. Ladd bands = fibrous peritoneal bands from cecum crossing the duodenum → duodenal obstruction
4. Small bowel clumped on right side of abdomen (diagnostic clue on imaging)
5. Bilious vomiting in a neonate = malrotation with volvulus until proven otherwise
6. Can lead to midgut volvulus → ischemia of entire midgut (SMA compromise)
7. Best diagnostic test = upper GI series (duodenojejunal junction displaced RIGHT)
8. Treatment = Ladd procedure (surgical emergency)
9. A "double bubble" sign = duodenal obstruction (also seen in duodenal atresia — differentiate by age/context)
10. Complication: complete midgut infarction if volvulus not corrected promptly

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2. INTUSSUSCEPTION

What It Is

Pathophysiology

• Proximal segment + its mesentery telescopes into distal lumen
• Mesenteric vessels are compressed → venous congestion first, then arterial occlusion
• Congested mucosa bleeds → mixes with mucus → "currant jelly" stools (dark red, mucus-laden)
• Progressive ischemia → necrosis → perforation if untreated

Lead Points — Critical Distinction

Associations

• IgA vasculitis (Henoch-Schönlein Purpura / HSP) — bowel wall edema/hemorrhage can act as lead point
• Adenovirus infection → Peyer patch hypertrophy → lead point (classic pediatric association)
• Lymphoma, polyps, lipomas (adults)

Clinical Presentation

Diagnosis

• Ultrasound (first-line in children): "Target sign" or "bull's-eye sign" — concentric rings of bowel on cross-section
• CT scan: also shows target sign (used more in adults)
• Plain X-ray: may show intestinal obstruction pattern

Treatment

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🔴 Step 1 MCQ High-Yield Points — Intussusception

1. Ileocecal junction is most common site
2. Classic triad: colicky abdominal pain + currant jelly stools + sausage-shaped RLQ mass
3. Child draws knees to chest to ease pain (reduces intra-abdominal pressure)
4. "Currant jelly" stools = blood + mucus from ischemic bowel wall (NOT fresh bright red blood)
5. Imaging: "Target sign" on ultrasound/CT = hallmark
6. Most common cause in children = idiopathic (adenovirus → Peyer patch hypertrophy)
7. Most common pathologic lead point in children = Meckel diverticulum
8. Most common lead point in adults = malignant tumor → always operate in adults
9. Associated with IgA vasculitis (HSP) — know this connection
10. Treatment in children (stable) = air/barium enema — therapeutic AND diagnostic
11. Treatment in adults = surgery + resection (assume malignancy)
12. Majority of cases in infants, unusual in adults (if adult → think tumor)

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Quick Comparison Table

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Connecting the Two Pages Together (Step 1 Integration)

• Meckel diverticulum → lead point for intussusception in children
• Hirschsprung disease → also presents with bilious emesis/obstruction in neonates (distinguish from malrotation by: meconium not passed vs. bilious vomiting; rectal biopsy vs. UGI series)
• Malrotation volvulus → also can present with bilious emesis like duodenal atresia (distinguish: atresia = "double bubble" on XR at birth, complete obstruction; malrotation = intermittent, UGI series shows corkscrew)