Built from GI Lecture 01 (LOs 1–7). Close notes. Fill from memory. Reveal only after a genuine attempt.

LO 1

ON EXAM — wall layers (inside→out), plexus locations, parietal/chief/G cells, Hirschsprung, skeletal vs smooth muscle

GI segments — fill in primary function for each (LO1)

Segment	Key Feature / Landmark	Primary Function
Stomach	Fundus, body, antrum; rugae; pyloric sphincter	Storage, mechanical mixing (chyme), acid digestion, pepsin activation
Duodenum	~25 cm; receives bile + pancreatic duct at ampulla of Vater	Neutralization of acid chyme; major digestion begins; CCK + secretin released here
Jejunum	~2.5 m; most villi/microvilli; most plicae circulares	Majority of nutrient absorption
Ileum	~3.5 m; Peyer's patches; ileocecal valve	Final reclamation: B12 absorption (requires intrinsic factor) + bile salt reabsorption
Colon	Ascending → transverse → descending → sigmoid; haustra, teniae coli	Water/electrolyte reabsorption; stool formation; Na⁺ absorbed, K⁺ secreted
Anal canal	Internal + external sphincters	Controlled defecation — internal sphincter = smooth/involuntary; external = skeletal/voluntary

Wall layers — name each layer inside → outside and what lives there (LO1)

Layer 1 (innermost)

Mucosa — epithelium + lamina propria (capillaries, lacteals, immune cells) + muscularis mucosae

Layer 2

Submucosa — dense CT; blood vessels, lymphatics; Meissner's (submucosal) plexus lives here

Layer 3 — two muscle sublayers + what plexus lives between them?

Muscularis externa: inner circular + outer longitudinal smooth muscle; Auerbach's (myenteric) plexus lies BETWEEN them

Layer 4 (outermost) — two names, when does each apply?

Serosa (intraperitoneal) / Adventitia (retroperitoneal)

Plexuses — the "My Sub" mnemonic (LO1)

Mnemonic: MYenteric = Motility (between circular & longitudinal muscle) / SUBmucosal = Secretion (between circular muscle & submucosa)

Plexus	Other Name	Location	Primary Function
Myenteric	Auerbach's plexus	Between circular and longitudinal muscle layers	Controls MOTILITY (peristalsis, segmentation)
Submucosal	Meissner's plexus	Between circular muscle and submucosa	Controls SECRETION and local mucosal blood flow

Key gastric cell types — fill product/function (LO1)

Cell Type	Location	Product / Function
Parietal cells	Gastric fundus/body	Secrete HCl (H⁺/K⁺ ATPase) AND intrinsic factor (required for B12 absorption in terminal ileum)
Chief cells	Gastric fundus/body	Secrete pepsinogen (inactive) → activated to pepsin at pH <3.5 by HCl
G cells	Gastric antrum + duodenum	Secrete gastrin → stimulates parietal cells → ↑HCl secretion
Goblet cells	Throughout SI and colon	Secrete mucus — lubricates and protects epithelium from acid and enzymes
Paneth cells	Crypt base (small intestine)	Secrete defensins + lysozyme → innate immunity; support crypt stem cell niche
Enteroendocrine cells	Throughout GI tract	Produce GI hormones into blood: CCK, secretin, GIP, GLP-1, somatostatin, gastrin

Surface area adaptations & skeletal muscle locations (LO1)

3 levels of surface area amplification (name each + fold increase)

1) Plicae circulares ×3 | 2) Villi ×10 | 3) Microvilli (brush border) ×20 → ~600-fold total

Where does the lacteal fit in this structure?

Lymphatic vessels inside each villus → absorb chylomicrons (dietary fats) → thoracic duct, bypassing portal vein

3 locations of skeletal (voluntary) muscle in GI tract

1) Mouth, pharynx, upper esophagus (voluntary swallowing) | 2) External anal sphincter (voluntary defecation)

Hirschsprung disease — what fails embryologically? What does the aganglionic segment DO?

Neural crest cells fail to migrate into distal colon → absent BOTH plexuses → tonic contraction → functional obstruction; proximal dilated segment has NORMAL ganglia

LO 2 · LO 3

ON EXAM — DEEP MASS (know all 8), hormones → blood vs enzymes → lumen, fluid volumes, pH gradient, vomiting vs diarrhea acid-base

DEEP MASS — expand each letter with mechanism (LO2)

D — Digestion (enzymatic)

Enzymatic breakdown: carbs → monosaccharides, proteins → amino acids, fats → fatty acids + monoglycerides

E — Elimination

Remove waste; gastric acid kills bacteria; MALT = immune defense; bacteria eliminated in stool

E — Endocrine function

GI tract = LARGEST endocrine organ; enteroendocrine cells secrete gastrin, CCK, secretin, GIP, GLP-1 INTO BLOOD

P — Protection/Immune

Mucosal barrier; IgA secretion; gastric acid kills pathogens; Peyer's patches (ileum) + MALT = adaptive immunity

M — Motility/Propulsion

Peristalsis moves content aborally; segmentation mixes content locally

A — Absorption

Nutrients, electrolytes, water absorbed — almost everything reabsorbed; fiber partially fermented by colonic bacteria

S — Secretion

Digestive enzymes, HCl, bile, bicarbonate secreted into lumen; ~7–8 L/day facilitates digestion

S — Storage

Stomach stores + releases chyme in controlled manner; colon stores stool while dehydrating it

Hormones vs. enzymes — where do they go? (LO2)

EXAM TRAP: GI hormones (gastrin, CCK, secretin, GIP, GLP-1) are ALWAYS secreted INTO BLOOD, not the lumen. Digestive enzymes (amylase, pepsin, lipase) go INTO THE LUMEN.

Gastrin — source, goes where, what it stimulates

G cells (antrum + duodenum) → into BLOOD → stimulates parietal cells → ↑HCl + pepsinogen

CCK — trigger, goes where, targets

Duodenal cells (FA + AA) → blood → gallbladder contraction + pancreatic enzyme secretion + opens sphincter of Oddi

Secretin — trigger, goes where, targets

Duodenal S cells (low pH) → blood → stimulates pancreatic HCO₃⁻ secretion to neutralize acid chyme

Pepsin — source, goes where, activation

Chief cells → INACTIVE pepsinogen INTO LUMEN → activated to pepsin by HCl (pH <3.5) in lumen

Fluid volumes — fill in numbers (LO3)

Source	Volume/Day	Key Components
Ingested fluids	~2 L	Water, electrolytes from diet
Saliva	~1.5 L	Amylase, mucin, IgA, bicarbonate; pH 6.0–7.0
Gastric juice	~2 L	HCl (pH 1.5–2.0), pepsinogen, intrinsic factor, mucus
Bile	~0.5 L	Bile salts, bilirubin, cholesterol, phospholipids; pH 7–8
Pancreatic juice	~1.5 L	Bicarbonate (neutralizes acid) + zymogens (trypsinogen, lipase, amylase)
Intestinal secretions	~1.5–2 L	Water, electrolytes, mucus from intestinal crypts
TOTAL entering SI	~9–10 L/day

Fluid absorption — where does all that fluid go? (LO3)

Small intestine absorbs how much?

~8,500 mL (8.5 L) — driven by Na⁺ co-transport with glucose/amino acids

Colon absorbs how much?

~400 mL — Na⁺ absorbed, K⁺ secreted; final stool dehydration

How much is excreted in feces?

~100–200 mL — bacteria + undigested fiber + sloughed epithelial cells

Clinical: cholera mechanism and why lethal

Cholera toxin → ↑cAMP → massive Cl⁻ secretion + water follows → >20 L/day stool → life-threatening dehydration

pH gradient along GI tract (LO3)

Segment	Approximate pH	What drives it
Mouth / Saliva	6.0–7.0	Bicarbonate in saliva
Stomach	1.5–2.0	HCl from parietal cells (H⁺/K⁺ ATPase)
Duodenum (proximal)	4.0–5.0	Acid chyme enters → secretin → pancreatic HCO₃⁻ neutralizes
Jejunum	6.0–7.0	Progressive neutralization by pancreatic HCO₃⁻
Ileum (terminal)	7.0–7.5	Bicarbonate accumulation
Colon	5.5–7.0	Bacterial fermentation → short-chain fatty acids lower pH

Vomiting → what acid-base change and why?

Metabolic ALKALOSIS — lose HCl → lose H⁺ and Cl⁻ → ↑HCO₃⁻; + hypokalemia

Diarrhea → what acid-base change and why?

Metabolic ACIDOSIS — lose HCO₃⁻-rich intestinal secretions; + hypokalemia (colon secretes K⁺)

Why pepsin works only in stomach

Pepsin active only at pH <3.5; PPIs ↑ stomach pH → deactivate pepsin → effective ulcer treatment

Enteric-coated drugs — target pH for dissolution

Dissolve at pH ≥5–6, targeting small intestine; gastric acid would destroy if uncoated

LO 4

ON EXAM — ENS autonomy, myenteric vs submucosal, excitatory (ACh/SubP) vs inhibitory (VIP/NO) neurons, ileus types, opioids + μ-receptor

ENS overview — the "second brain" (LO4)

How many neurons in the ENS? Compare to spinal cord.

~500 million neurons — MORE than in the entire spinal cord

Can the GI tract function WITHOUT the brain or spinal cord?

YES — semi-autonomous; denervated transplanted intestinal segments still peristalse; spinal cord injury patients retain basal GI motility

What is the role of extrinsic ANS (vagus + sympathetics) relative to ENS?

ANS MODULATES but does not drive basic function — amplifies (PSNS) or suppresses (SNS) ENS; ENS = 'local government'; ANS = 'federal government'

What happens if you remove ENS entirely (vs. remove extrinsic innervation)?

Remove ENS → ILEUS (no peristalsis at all) | Remove extrinsic only → less coordinated but still functional gut

Two plexuses — comparison table (LO4)

Feature	Myenteric Plexus (Auerbach's)	Submucosal Plexus (Meissner's)
Location	Between circular and longitudinal muscle layers	Between circular muscle and submucosa
Primary function	MOTILITY — controls peristalsis and segmentation	SECRETION — controls secretion + mucosal blood flow
Distribution	Entire GI tract (esophagus to rectum)	Mainly small + large intestine (less in esophagus/stomach)
Stimulus it responds to	Stretch (mechanoreceptors) → initiates peristalsis	Chemical signals → triggers secretion
Absent in Hirschsprung?	YES — both plexuses absent in aganglionic segment	YES — both absent

ENS neuron types (LO4)

Excitatory motor neurons — neurotransmitters (2)

ACh + Substance P → contract smooth muscle; drive ASCENDING EXCITATION proximal to bolus

Inhibitory motor neurons — neurotransmitters (2)

VIP + NO → relax smooth muscle; drive DESCENDING INHIBITION distal to bolus

Sensory (afferent) ENS neurons — what do they detect?

Detect stretch, chemicals, osmolarity → feed into ENS reflex arcs; also signal CNS via vagal afferents

Interneurons in ENS — role

Integrate signals; form local reflex circuits; coordinate ascending excitation and descending inhibition

Ileus — types and causes (LO4)

KEY: Ileus = absence of GI motility. ENS cannot function → peristalsis ceases. Three distinct types — know the mechanism of each.

Type	Cause	Mechanism
Paralytic (adynamic)	Post-surgery, peritonitis, opiates, hypokalemia	SNS overactivation or smooth muscle inhibition → ENS inhibited → no peristalsis; absent bowel sounds
Hirschsprung	Absent ganglia (failure of neural crest migration)	No ENS neurons → tonic contraction of aganglionic segment → functional obstruction; neonatal failure to pass meconium
Mechanical obstruction	Adhesions, hernia, tumor, volvulus	Physical block; motility may be exaggerated proximal before eventually failing

Opioid mechanism of constipation/ileus — receptor + location

Bind μ-opioid receptors on ENS neurons → inhibit ACh + Substance P release → ↓peristalsis + ↑sphincter tone

Treatment for opioid-induced constipation without reversing analgesia

Methylnaltrexone (Relistor) — peripherally-acting μ-opioid antagonist; does NOT cross BBB → blocks GI without reversing CNS analgesia

LO 5

ON EXAM — 3 receptor types + clinical correlates, peristaltic reflex (ascending ACh/SubP, descending VIP/NO), serotonin 90% in GI, 5-HT3 antagonists vs 5-HT4 agonists, achalasia, VIPoma

Three GI sensory receptor types (LO5)

1. Mechanoreceptors

Stimulus

Distension/stretch of GI wall (intraluminal pressure or volume)

Primary response

Primary trigger for peristaltic reflex; over-distension → vomiting reflex; gastric distension → satiety via vagal afferents

Clinical: post-op ileus mechanism

Surgical bowel handling stuns mechanoreceptors → ENS cannot initiate reflex → paralytic ileus even with intact ENS neurons

2. Chemoreceptors

Stimulus

Chemical composition of lumen: pH, fatty acids, amino acids, glucose, bile acids, bacterial products

Primary response

Trigger hormone secretion: CCK (FA + AA in duodenum) + secretin (low pH) → downstream pancreatic/biliary effects

Clinical: secretin release trigger + effect

Low intraluminal pH (acid chyme) in duodenum → secretin → pancreatic HCO₃⁻ to neutralize and protect duodenum

3. Osmoreceptors

Stimulus

Osmolarity of luminal contents — most sensitive in duodenum and jejunum

Primary response

Hyperosmolar chyme → slows gastric emptying; triggers fluid secretion into lumen to dilute content before absorption

Clinical: dumping syndrome mechanism

Post-gastrectomy: rapid emptying of hyperosmolar chyme overwhelms osmoreceptors → fluid rushes in → hypovolemia + nausea + diarrhea

Key GI neurotransmitters — fill effects and associations (LO5)

Neurotransmitter	Effect on GI	High-Yield Association / Clinical
ACh (excitatory)	↑Motility (peristalsis); ↑secretion; contracts smooth muscle via muscarinic M3 receptor	Atropine (muscarinic blocker) → ↓motility; vagal activation → ACh → ↑gastric acid
Substance P (excitatory)	↑Smooth muscle contraction; co-released with ACh in ascending (excitatory) limb of peristaltic reflex	Primary sensory afferent NT; ascending excitation proximal (oral side) of bolus
VIP (inhibitory)	↓Smooth muscle (relaxation); ↑secretion; vasodilation — drives DESCENDING INHIBITION	VIPoma → massive watery diarrhea = WDHA syndrome (Verner-Morrison); VIP/NO loss at LES = achalasia
Nitric Oxide NO (inhibitory)	↓Smooth muscle tone; maintains LES and pyloric sphincter relaxed between contractions	Loss of NO neurons at LES → achalasia (LES fails to relax); bird-beak on barium swallow
Serotonin / 5-HT	Initiates peristaltic + secretory reflexes via 5-HT3 and 5-HT4 receptors on ENS neurons	90% of body 5-HT in enterochromaffin cells; 5-HT3 antagonists = antiemetics; 5-HT4 agonists = prokinetics; carcinoid → right-sided heart disease
Neuropeptide Y (NPY)	↓Motility; ↓secretion; splanchnic vasoconstriction — mediates sympathetic suppression of GI	Released by sympathetic neurons; mediates SNS gut-shutdown during fight-or-flight

Peristaltic reflex — step by step (LO5)

Step 1 — stimulus that triggers the reflex

Distension OR chemical stimuli → enterochromaffin cells release 5-HT → activates IPANs via 5-HT3 receptors

Step 2 — ascending excitation (proximal/oral side of bolus)

ACh + Substance P → circular muscle CONTRACTS above (proximal/oral side of) bolus → propels content forward

Step 3 — descending inhibition (distal/aboral side of bolus)

VIP + NO → circular muscle RELAXES below (distal/aboral side of) bolus → creates space for content

Achalasia — which limb fails and why?

DESCENDING INHIBITION fails — VIP/NO neurons at LES destroyed (autoimmune) → LES cannot relax → bird-beak dilation; Tx: botulinum toxin, pneumatic dilation, or Heller myotomy

Serotonin — what % of body total is in GI? Which cells produce it?

~90% of body's total serotonin; produced by enterochromaffin (EC) cells in intestinal mucosa

5-HT3 antagonists — drug example + clinical use

Ondansetron, granisetron — block 5-HT3 on vagal afferents → antiemetic; used for chemotherapy-induced and post-op nausea

5-HT4 agonists — drug example + clinical use

Cisapride, prucalopride — ↑ACh release → prokinetic; used for gastroparesis and chronic constipation

Carcinoid syndrome — mechanism and why RIGHT-sided heart disease only

EC cell tumor → massive 5-HT → liver overwhelmed → systemic 5-HT; right-sided heart fibrosis (tricuspid/pulmonary); pulmonary vasculature inactivates 5-HT → left heart spared; urine 5-HIAA elevated

LO 6

ON EXAM — Celiac/SMA/IMA territories (embryologic origin), watershed zones, portal vein route, lipid exception (lacteals → thoracic duct), portal hypertension consequences

Three GI arteries — fill territories (LO6)

Mnemonic: embryologic — Foregut = Celiac | Midgut = SMA | Hindgut = IMA. Watershed zones are where two supplies border each other → vulnerable to ischemia.

Artery	Embryologic Origin	Structures Supplied (fill in)
Celiac artery	Foregut	Distal esophagus, stomach, duodenum (1st + 2nd parts), liver, gallbladder, spleen, head of pancreas
Superior Mesenteric (SMA)	Midgut	Duodenum (distal 3rd + 4th parts), jejunum, ileum, cecum, ascending colon, proximal 2/3 transverse colon
Inferior Mesenteric (IMA)	Hindgut	Distal 1/3 transverse colon, descending colon, sigmoid colon, upper rectum

Watershed zone #1 — where is it and WHY is it vulnerable?

Splenic flexure — border between SMA and IMA territories → most vulnerable to ischemic colitis

Watershed zone #2

Rectosigmoid junction — border between IMA and pudendal/hemorrhoidal supply → second most vulnerable

SMA embolism — classic clinical presentation (Step 1 clue)

Sudden-onset severe periumbilical pain OUT OF PROPORTION to exam + atrial fibrillation → surgical emergency; visceral pain precedes peritoneal signs

Celiac artery 3 main branches

1) Left gastric artery (stomach) | 2) Common hepatic artery (→ liver + gastroduodenal) | 3) Splenic artery (spleen + pancreatic branches)

Portal circulation & first-pass metabolism (LO6)

Portal vein route — trace the path

GI capillaries → portal vein → hepatic sinusoids → hepatic veins → IVC → right heart → systemic circulation

Which nutrients go via portal vein? (water-soluble route)

Glucose, amino acids, short-chain FAs, water-soluble vitamins → portal vein → LIVER FIRST-PASS before systemic

The lipid exception — how do fats bypass the liver?

Long-chain FAs + ADEK → chylomicrons → lacteals → thoracic duct → left subclavian vein → BYPASSES liver entirely

Clinical significance of lipid bypass (2 points)

1) Fat malabsorption → ADEK deficiency | 2) Lipophilic drugs in lipid carriers can bypass liver first-pass → ↑bioavailability

Portal hypertension consequences (LO6)

Cirrhosis → ↑ resistance in hepatic sinusoids → portal pressure rises → portosystemic shunts open

Portosystemic Shunt	Location	Clinical Consequence
Esophageal varices	Left gastric vein ↔ esophageal submucosal veins ↔ azygos vein	MOST DANGEROUS — rupture → massive, potentially fatal upper GI hemorrhage
Caput medusae	Paraumbilical veins (recanalization of umbilical vein)	Visible dilated superficial veins radiating from umbilicus — cosmetic sign of portal HTN
Hemorrhoids (rectal varices)	Superior rectal (portal) ↔ middle/inferior rectal (systemic)	Rectal varices / hemorrhoids — less immediately dangerous than esophageal varices

LO 7

ON EXAM — Vagus CN X territory (esophagus → transverse colon), pelvic nerve (S2–S4), SNS origin T5–L2, prevertebral ganglia, cephalic phase 20–30%, 3 phases of acid secretion

Parasympathetic — "rest and digest" (LO7)

Feature	Vagus Nerve (CN X)	Pelvic Nerve (S2–S4)
GI Territory	Esophagus → transverse colon (~75% of GI tract)	Distal transverse colon, descending colon, sigmoid colon, rectum
Preganglionic fiber	Long; travels in vagus nerve to ENS ganglia in GI wall	Long; in pelvic splanchnic nerves to ENS ganglia in distal colon/rectum
Postganglionic NT	ACh (muscarinic)	ACh (muscarinic)
Effect on GI	↑Motility, ↑secretion, ↑gastric acid, ↑enzyme secretion; mediates cephalic phase	↑Motility in distal colon; coordinates defecation reflex

Step 1 rule: PSNS GI question anywhere from esophagus to transverse colon → what nerve?

VAGUS (CN X) — carries PSNS to esophagus through transverse colon; pelvic nerve (S2–S4) for distal colon/rectum

PSNS ganglia location in GI — preganglionic vs postganglionic fiber lengths

Ganglia are IN the GI wall (ENS ganglia) → long preganglionic + short postganglionic

Sympathetic — "fight or flight" (LO7)

Feature	Detail
Spinal cord origin	T5–L2
Preganglionic route	Splanchnic nerves (greater, lesser, least, lumbar) → prevertebral ganglia
Prevertebral ganglia (3)	Celiac ganglion (foregut) + Superior mesenteric ganglion (midgut) + Inferior mesenteric ganglion (hindgut)
Postganglionic NT	Norepinephrine (NE) + Neuropeptide Y (NPY)
Effect on motility	↓Peristalsis (↓ENS excitability); ↑sphincter tone (contracts IAS and ileocecal valve)
Effect on secretion	↓Gastric acid, pancreatic enzymes, bile — shuts down digestion
Effect on blood flow	Splanchnic vasoconstriction → blood diverted to muscles, brain, heart

Cephalic phase (LO7)

The cephalic phase occurs BEFORE food enters the GI tract — triggered purely by CNS anticipation. Mediated by vagal efferents.

Stimuli for cephalic phase

Sight, smell, taste, or THOUGHT of food → cortical + hypothalamic → dorsal motor nucleus of vagus → vagal efferents → ENS

Effects of cephalic phase activation

↑Salivation, ↑HCl, ↑pepsinogen, ↑gastric motility, ↑pancreatic enzyme secretion — before food arrives

What % of total gastric acid response is cephalic phase?

20–30% of total meal-stimulated gastric acid response

Fill in the 3 phases of gastric acid secretion + % each

1) CEPHALIC (20–30%): sight/smell → vagal ACh → ↑HCl | 2) GASTRIC (60–70%): food → distension + AA/peptides → gastrin | 3) INTESTINAL (5–10%): chyme in duodenum; CCK/secretin/GIP dampen acid

SNS vs PSNS — quick comparison (LO7)

Effect	PSNS (Vagus / Pelvic)	SNS (Splanchnic nerves)
GI motility	↑ (amplifies ENS peristalsis)	↓ (inhibits ENS, slows peristalsis)
Secretion	↑ (gastric acid, enzymes, salivation)	↓ (shuts down all secretion)
Sphincter tone	↓ (sphincters relax to allow flow)	↑ (sphincters CONTRACT — holds content back)
Splanchnic blood flow	↑ post-prandial hyperemia (25–30% of CO at rest, more after meal)	↓ splanchnic vasoconstriction → blood diverted to muscles/brain
Ganglia location	IN the GI wall (ENS ganglia) → long preganglionic, short postganglionic	PREVERTEBRAL ganglia (celiac, SMA, IMA) → short preganglionic, long postganglionic

LO 1–7 · USMLE Step 1 Style

8 questions — cover all 7 LOs. Use these after finishing all tabs.

Q1. A histology slide shows neurons located between the inner circular and outer longitudinal smooth muscle layers of the small intestine. Damage to these neurons would MOST directly impair which function?

A. Mucosal enzyme secretion and local blood flow
B. Coordinated peristalsis and segmentation
C. IgA production and mucosal immune defense
D. Absorption of long-chain fatty acids into lacteals
E. Pancreatic bicarbonate release in response to acid chyme

B — Coordinated peristalsis and segmentation
The myenteric (Auerbach's) plexus lies between the circular and longitudinal muscle layers and controls MOTILITY. The submucosal (Meissner's) plexus, located between circular muscle and submucosa, controls secretion and blood flow (choice A). Choices C–E involve immune, absorptive, and hormonal functions not directly controlled by this plexus location.

Q2. A 3-day-old neonate fails to pass meconium and develops progressive abdominal distension. Rectal biopsy reveals an absence of ganglion cells in the distal rectosigmoid. What embryological event failed, and what would you find in the PROXIMAL dilated segment on biopsy?

A. Failed mesodermal differentiation; absent muscularis mucosae proximally
B. Failed neural crest migration; normal ganglion cells present proximally
C. Failed neural crest migration; absent ganglion cells throughout the colon
D. Defective Wnt signaling; hyperplastic crypts proximally
E. Failed endodermal rotation; volvulus with normal ganglia throughout

B — Failed neural crest migration; normal ganglion cells present proximally
Hirschsprung disease = failure of neural crest cells to migrate into the distal colon during embryogenesis → aganglionic segment → tonic contraction (functional obstruction). The proximal dilated segment has NORMAL ganglia (it dilates because stool cannot pass through the contracted distal segment). This is a classic neonatal surgical emergency.

Q3. A patient is found to have metabolic alkalosis and hypokalemia. Which of the following processes is the MOST likely cause?

A. Profuse watery diarrhea from Vibrio cholerae
B. Prolonged nasogastric suctioning of gastric contents
C. Secretory diarrhea from a VIPoma
D. Pancreatic fistula draining into the peritoneum
E. Ileostomy with high output from the small intestine

B — Prolonged nasogastric suctioning of gastric contents
Loss of gastric fluid (HCl) → loss of H⁺ and Cl⁻ → metabolic ALKALOSIS + hypokalemia (K⁺ shifts into cells as H⁺ exits, and ↑aldosterone from volume depletion causes renal K⁺ wasting). All other choices cause diarrhea → loss of HCO₃⁻-rich intestinal/pancreatic fluid → metabolic ACIDOSIS + hypokalemia.

Q4. A patient on long-term IV opioids for cancer pain develops severe constipation and abdominal distension. The team wants to restore bowel function without reversing analgesia. Which of the following is the BEST option?

A. Naloxone IV (systemic opioid antagonist)
B. Metoclopramide (5-HT4 agonist / D2 antagonist)
C. Methylnaltrexone (peripherally-acting μ-opioid antagonist)
D. Atropine (muscarinic antagonist)
E. Ondansetron (5-HT3 antagonist)

C — Methylnaltrexone (peripherally-acting μ-opioid antagonist)
Opioids bind μ-receptors on ENS neurons → inhibit excitatory NT (ACh/Substance P) → ↓peristalsis + ↑sphincter tone. Methylnaltrexone does NOT cross the BBB, so it blocks the GI μ-receptors without reversing CNS analgesia. Naloxone (A) crosses BBB → would reverse analgesia. Metoclopramide (B) is a prokinetic but does not address the receptor-level cause. Atropine (D) would WORSEN constipation.

Q5. A 68-year-old patient with atrial fibrillation presents with sudden-onset severe periumbilical pain for 2 hours. His abdomen is soft on exam. CT angiography shows an embolus at the origin of the superior mesenteric artery. Which bowel segments are at GREATEST risk for ischemia?

A. Esophagus, stomach, and proximal duodenum
B. Descending colon, sigmoid colon, and upper rectum
C. Distal esophagus, liver, spleen, and gallbladder
D. Jejunum, ileum, cecum, ascending colon, and proximal transverse colon
E. Distal transverse colon, descending colon, and sigmoid colon only

D — Jejunum, ileum, cecum, ascending colon, and proximal transverse colon
The SMA supplies the MIDGUT: duodenum (distal 3rd/4th parts), jejunum, ileum, cecum, ascending colon, and proximal 2/3 of transverse colon. The "pain out of proportion to exam" is the classic Step 1 clue — ischemia causes severe visceral pain mediated by mesenteric afferents before transmural necrosis and peritoneal signs develop. AF = embolic source. Choice A = celiac territory; Choice B/E = IMA; Choice C = celiac.

Q6. A patient develops progressive dysphagia to both solids and liquids. Esophageal manometry reveals failure of the lower esophageal sphincter to relax during swallowing and absence of peristalsis in the distal esophagus. Barium swallow shows a "bird-beak" narrowing at the gastroesophageal junction. Which neurotransmitter deficit BEST explains the pathophysiology?

A. Loss of excitatory ACh + Substance P neurons at LES
B. Excess VIP release causing sustained smooth muscle relaxation
C. Loss of inhibitory VIP/NO neurons at LES → unopposed excitatory tone
D. Excess Substance P causing hypertensive peristalsis
E. Loss of 5-HT3 receptor signaling in ENS afferents

C — Loss of inhibitory VIP/NO neurons at LES → unopposed excitatory tone
Achalasia = selective destruction of inhibitory ENS neurons (nNOS/VIP-positive) at the LES. LES relaxation requires VIP + NO (descending inhibitory limb). Without inhibitory input, unopposed ACh/Substance P excitatory tone → LES stays contracted → food cannot enter stomach → progressive esophageal dilation. Treatments: botulinum toxin (blocks ACh to rebalance), pneumatic dilation, or surgical myotomy (Heller).

Q7. A patient with cirrhosis presents with hematemesis. Endoscopy reveals bleeding esophageal varices. Which statement BEST explains the anatomical basis for variceal formation?

A. Splanchnic arterial vasodilation increases hepatic arterial blood flow
B. Portal hypertension forces blood into portosystemic anastomoses between the left gastric vein and azygos veins
C. Hepatic venous outflow obstruction causes reversal of portal venous flow
D. Inferior mesenteric vein hypertension causes bleeding at the rectosigmoid junction
E. Chylomicron accumulation in the thoracic duct causes esophageal dilation

B — Portal hypertension forces blood into portosystemic anastomoses between the left gastric vein and azygos veins
Cirrhosis → ↑hepatic sinusoidal resistance → portal hypertension → blood backs up into portal tributaries → portosystemic shunts open. At the esophagus: left gastric (portal) ↔ esophageal submucosal veins ↔ azygos vein (systemic). These thin-walled esophageal varices are at high risk of rupture → most dangerous portal HTN complication. Other shunts: caput medusae (paraumbilical veins) and hemorrhoids (superior rectal ↔ inferior/middle rectal).

Q8. A patient smells a meal being prepared and begins salivating with audible stomach growling before eating anything. What is the physiologic name for this phenomenon, and what percentage of total meal-stimulated gastric acid production does it account for?

A. Gastric phase; 60–70% of acid response
B. Intestinal phase; 5–10% of acid response
C. Cephalic phase; 20–30% of acid response
D. Enteric phase; 40–50% of acid response
E. Cephalic phase; 60–70% of acid response

C — Cephalic phase; 20–30% of acid response
The cephalic phase occurs before food even enters the GI tract, triggered by sight/smell/taste/thought of food → cortical + hypothalamic activation → dorsal motor nucleus of vagus → vagal efferents → ENS → ↑HCl, pepsinogen, salivation, and gastric motility. Accounts for ~20–30% of total acid response. Gastric phase (distension + amino acids → gastrin) accounts for 60–70%. Intestinal phase (chyme in duodenum) = 5–10%. Pavlov's "psychic secretion" experiments demonstrated this in dogs.

GI Overview & Physiology — Answer Key