Instructions: Read each clinical vignette carefully. Select the single best answer (A–E) for each question. Your responses are tracked in the Score Report tab. Target completion: 27 minutes (~90 seconds per question). Good luck!
Question 1 · Gastric Acid Secretion
A 42-year-old male presents with a 3-year history of recurrent duodenal ulcers refractory to standard acid-suppression therapy. He reports chronic watery diarrhea, unintentional weight loss, and epigastric pain. Physical exam is unremarkable. Fasting serum gastrin is 2,500 pg/mL (normal <100). Basal acid output is markedly elevated at 35 mEq/hr. A secretin stimulation test paradoxically increases gastrin by 200 pg/mL. Upper endoscopy shows multiple duodenal ulcers and prominent gastric folds.
What is the most likely diagnosis?
A
H. pylori infection with reactive gastrin elevation
B
NSAID-induced gastropathy with secondary acid hypersecretion
C
Gastrinoma (Zollinger-Ellison syndrome)
D
Pernicious anemia with achlorhydria-induced compensatory gastrin rise
E
Ménétrier disease with hypertrophic gastric folds
Answer: C — Gastrinoma (Zollinger-Ellison syndrome)
The key diagnostic triad is: (1) severe, refractory acid hypersecretion causing recurrent/atypical ulcers; (2) elevated fasting gastrin >1000 pg/mL; (3) paradoxical gastrin increase on secretin stimulation (>200 pg/mL rise indicates gastrinoma). Gastrinomas are gastrin-secreting neuroendocrine tumors (usually in the "gastrinoma triangle" of the pancreas or duodenum) that constitutively release gastrin, stimulating parietal cells → massive HCl production → severe ulcer disease and diarrhea (from acid inactivating pancreatic enzymes). The secretin paradox occurs because gastrinoma cells have aberrant secretin receptors coupled to gastrin release. Why other options fail: H. pylori causes mild gastrin elevation (<300) and fasting acid would normalize with treatment; NSAID gastropathy presents with normal gastrin and negative secretin test; pernicious anemia causes achlorhydria (low acid), not hypersecretion; Ménétrier disease shows elevated gastrin but normal acid secretion and no secretin response.
Question 2 · H. pylori Pathophysiology
A 38-year-old male with a 2-year history of recurrent epigastric pain relieved by meals and antacids presents with positive urea breath test and H. pylori PCR on gastric biopsy. Serum gastrin level is mildly elevated at 180 pg/mL. Antral biopsy shows chronic lymphocytic infiltration with foveolar hyperplasia.
What is the primary mechanism by which H. pylori produces duodenal ulcer disease?
A
Direct activation of parietal cell acid secretion via bacterial toxin
B
Damage to antral D cells → ↓somatostatin → ↑gastrin → ↑HCl
C
CagA protein downregulates gastrin gene expression
D
Transformation to MALT lymphoma with T-cell driven inflammation
E
Disruption of mucus/HCO₃⁻ barrier → increased acid back-diffusion only
Answer: B — Damage to antral D cells → ↓somatostatin → ↑gastrin → ↑HCl
H. pylori colonizes the antrum and damages somatostatin-secreting D cells via inflammatory cytokines and bacterial urease, reducing negative-feedback inhibition of gastrin-secreting G cells. Unopposed G-cell stimulation elevates gastrin, driving parietal cells to hypersecrete HCl, which overwhelms mucosal defenses → duodenal ulcer. This occurs in the setting of intact acid secretory capacity (unlike gastrinoma's massive hypersecretion). Why other options fail: H. pylori does not directly activate parietal cells; CagA actually enhances gastrin production (not downregulates); MALT lymphoma is a downstream consequence, not the primary DU mechanism; while mucus barrier disruption contributes, the dominant mechanism is gastrin-driven acid hypersecretion.
Question 3 · B12 Malabsorption Mechanism
A 65-year-old female with a 1-year history of progressive fatigue, paresthesias, and gait disturbance is found on labs to have macrocytic anemia (MCV 112 fL), hypersegmented neutrophils on blood smear, low serum B12 (140 pg/mL), and high methylmalonic acid. Serum anti-intrinsic factor (anti-IF) antibodies and antiparietal cell antibodies are both positive. Gastric pH is 7.2. Upper endoscopy shows chronic gastritis with atrophic mucosa.
What is the mechanism of B12 deficiency in this patient?
A
↓Gastric acid → ↓pepsin → intact R-protein, competing with B12 for binding
B
Loss of intrinsic factor → B12 cannot be bound/absorbed in terminal ileum
C
Terminal ileal inflammation (Crohn's) → impaired B12 transporter expression
D
Folate deficiency → impaired B12-dependent methylation cycle
E
Iron deficiency → reduced transport of B12-IF complex
Answer: B — Loss of intrinsic factor → B12 cannot be bound/absorbed in terminal ileum
Pernicious anemia is autoimmune destruction of gastric parietal cells (anti-IF and antiparietal antibodies) → loss of intrinsic factor (IF) secretion → B12 cannot form the IF-B12 complex required for active absorption via cubilin receptors in the terminal ileum. Without IF, even dietary B12 remains unabsorbed. The elevated pH reflects achlorhydria due to loss of acid-secreting parietal cells. Why other options fail: While low acid does impair initial B12 release from R-protein binding, the dominant defect here is IF loss (confirmed by antibodies); terminal ileitis is not present; folate and iron deficiency are separate issues; this is B12 malabsorption, not transport failure per se.
Question 4 · Proton Pump Inhibitor Pharmacology
A 50-year-old male with GERD refractory to H₂-blockers is started on omeprazole 20 mg daily. Within 3 days, his symptoms improve dramatically. You explain to him the mechanism of action.
Which statement best describes omeprazole's mechanism?
A
Competitive blockade of histamine H₂ receptors on parietal cells
B
Irreversible inhibition of COX-1 in gastric mucosa
C
Irreversible inhibition of H⁺/K⁺-ATPase via prodrug activation in acidic canaliculus
D
Competitive antagonism of gastrin receptors (CCK-B) on parietal cells
E
Inhibition of carbonic anhydrase → reduced HCO₃⁻ generation in mucosa
Answer: C — Irreversible inhibition of H⁺/K⁺-ATPase via prodrug activation in acidic canaliculus
Omeprazole is a prodrug: it enters the parietal cell as inactive, crosses into the acidic secretory canaliculus, and is protonated to an active form. This reactive intermediate irreversibly sulfenamide-bonds to cysteine residues on H⁺/K⁺-ATPase ("proton pump"), blocking final acid secretion. Because the inhibition is irreversible, duration of action outlasts drug half-life (24+ hr despite 1 hr half-life), and new HCl synthesis requires new pump synthesis. Why other options fail: H₂-blockers are reversible competitors, not PPIs; PPIs don't inhibit COX or gastrin receptors; carbonic anhydrase inhibitors (acetazolamide) do not block acid secretion directly.
Question 5 · Sjögren Syndrome & Salivary Gland Pathology
A 55-year-old female with progressive dry mouth and dry eyes over 2 years presents with mild parotid gland swelling, difficulty swallowing, and recurrent mouth infections. Anti-SSA/Ro antibodies and anti-SSB/La antibodies are both elevated. Labial salivary gland biopsy shows lymphocytic infiltration with focal myoepithelial islands (Mikulicz bodies) and acinar atrophy.
What is the primary histopathologic finding causing xerostomia?
A
Sialolith obstruction in salivary duct
B
Mumps viral infection causing acute parotitis
C
Autoimmune lymphocytic infiltration and destruction of acini
D
Bacterial sialadenitis with abscess formation
E
Warthin tumor (papillary cystadenoma lymphomatosum)
Answer: C — Autoimmune lymphocytic infiltration and destruction of acini
Sjögren syndrome is an autoimmune disorder (anti-SSA/SSB antibodies) targeting salivary and lacrimal glands. T cells and B cells infiltrate and destroy the secretory acini → loss of salivary production → xerostomia. Biopsy shows lymphocytic foci and Mikulicz bodies (preserved myoepithelial islands surrounded by lymphocytes). The combination of bilateral parotid swelling, positive anti-SSA/SSB, and histologic acinar atrophy is pathognomonic. Why other options fail: Sialoliths present acutely with pain and obstruction; mumps causes acute viral parotitis, not chronic destruction; bacterial sialadenitis presents with fever, pus, not autoimmune markers; Warthin tumors are benign neoplasms, not destructive inflammation.
Question 6 · Cystic Fibrosis & Pancreatic Insufficiency
An 18-year-old male with cystic fibrosis (confirmed CFTR mutation) presents with chronic cough, steatorrhea, and low BMI (19). Sweat chloride test was 98 mEq/L (diagnostic). Fecal elastase-1 is 85 U/g (normal >200), and 72-hour fecal fat is 14 g/day (normal <7). Pancreatic imaging shows fatty infiltration and ductal dilation. Serum amylase and lipase are normal.
What is the primary GI mechanism of steatorrhea in CF?
A
Autoimmune acinar destruction (similar to autoimmune pancreatitis)
B
CFTR mutation → thick pancreatic secretions → ductal obstruction → acinar atrophy → exocrine insufficiency
C
Celiac disease with villous atrophy and nutrient malabsorption
D
Bile acid deficiency due to impaired ileoterminal absorption
E
Lactase deficiency causing osmotic diarrhea with secondary steatorrhea
Answer: B — CFTR mutation → thick pancreatic secretions → ductal obstruction → acinar atrophy → exocrine insufficiency
The CF transmembrane conductance regulator (CFTR) normally channels Cl⁻ into ducts. Mutated CFTR impairs Cl⁻/HCO₃⁻ secretion → viscous pancreatic juices accumulate → progressive duct obstruction → pancreatic autodigestion (inspissated secretions trap digestive enzymes) → chronic pancreatitis → acinar atrophy and fibrosis (fatty infiltration on imaging). Result: low fecal elastase and steatorrhea despite normal enzymes in serum. Why other options fail: Autoimmune pancreatitis has different etiology and imaging; celiac would show villous atrophy, not pancreatic fatty infiltration; bile acid malabsorption is a secondary issue; lactase deficiency causes osmotic diarrhea, not steatorrhea.
Question 7 · Cholera Enterotoxin Mechanism
A 30-year-old female returned 3 days ago from a trip to coastal Bangladesh and now presents with 2 days of massive watery diarrhea (>10 liters/day), severe dehydration, hypernatremia, and hypokalemia. Stool culture grows Vibrio cholerae. Serum electrolytes show hypokalemia and elevated bicarbonate. Small intestinal biopsy shows intact epithelium with normal villous architecture.
What is the molecular mechanism of cholera toxin-induced secretory diarrhea?
A
Gi activation → ↓cAMP → reduced Cl⁻ secretion
B
ADP-ribosylation of Gs → ↑cAMP → CFTR permanently open → massive Cl⁻ & H₂O secretion
C
GPCR internalization blocking intestinal hormone receptors
D
Direct covalent phosphorylation of CFTR by bacterial kinase
E
Enterocyte apoptosis → loss of tight junctions → osmotic diarrhea
Answer: B — ADP-ribosylation of Gs → ↑cAMP → CFTR permanently open → massive Cl⁻ & H₂O secretion
Cholera toxin's A subunit catalyzes NAD⁺-dependent ADP-ribosylation of the alpha subunit of Gs proteins, locking them in the GTP-bound (active) state → constitutive adenylyl cyclase activation → sustained cAMP elevation. High cAMP phosphorylates and permanently opens CFTR chloride channels in intestinal crypt cells → massive Cl⁻ efflux → osmotic H₂O follows → profuse "rice-water" diarrhea. Normal histology confirms the mechanism is secretory (not inflammatory/destructive). Why other options fail: Gi activation would decrease cAMP (opposite effect); GPCR internalization doesn't explain sustained secretion; phosphorylation is not reversible like ADP-ribosylation; apoptosis would show histologic damage.
Question 8 · VIPoma & Watery Diarrhea Syndrome
A 52-year-old male with a 1-year history of severe chronic diarrhea (8–12 L/day) presents with profound hypokalemia (K 2.8 mEq/L), dehydration, and hypochlorhydria. Secretory diarrhea is confirmed (stool osmolal gap <50). Fasting gastrin is low at 45 pg/mL. Serum VIP is markedly elevated at 750 pg/mL (normal <75). CT abdomen shows a 3 cm islet cell tumor in the pancreatic body.
Which hormone is responsible for the diarrhea in this patient?
A
Gastrin — driving acid hypersecretion and diarrhea
B
Serotonin — causing flushing and carcinoid diarrhea
C
Vasoactive intestinal peptide (VIP) — causing watery diarrhea & electrolyte loss
D
Somatostatin — inhibiting fluid secretion (paradoxical here)
E
Cholecystokinin (CCK) — stimulating pancreatic enzyme secretion
Answer: C — Vasoactive intestinal peptide (VIP) — causing watery diarrhea & electrolyte loss
VIPomas secrete large amounts of VIP, which is a potent secretagogue for intestinal crypt cells (similar to cholera toxin mechanism via cAMP). VIP causes massive watery secretory diarrhea with low fecal osmolal gap, profound K⁺ and Cl⁻ wasting, and hypochlorhydria (VIP inhibits gastrin and acid). The clinical syndrome "WDHA" (Watery Diarrhea, Hypokalemia, Achlorhydria) is pathognomonic for VIPoma. The markedly elevated serum VIP (750 pg/mL) with low gastrin excludes gastrinoma. Why other options fail: Gastrin causes acid hypersecretion, not hypochlorhydria; serotonin-secreting carcinoids cause flushing and diarrhea but would show elevated chromogranin and 5-HIAA; somatostatin suppresses secretion (opposite); CCK stimulates pancreatic enzymes, not diarrhea.
Question 9 · Carcinoid Syndrome & Hepatic Metabolism
A 58-year-old female with small intestinal carcinoid tumor and liver metastases presents with flushing, diarrhea, and new-onset tricuspid regurgitation. Urine 5-hydroxyindoleacetic acid (5-HIAA) is markedly elevated at 45 mg/24 hr (normal <8). Serum chromogranin A is 580 ng/mL (normal <100). She has mild systemic hypotension and evidence of right ventricular hypertrophy on echocardiogram.
Why does carcinoid syndrome develop now, despite the primary tumor being small?
A
Histamine alone is released by the primary tumor (serotonin is inert)
B
Serotonin from EC-cell tumor; liver mets bypass hepatic MAO → systemic serotonin
C
Pancreatic metastases causing VIP hypersecretion
D
Norepinephrine release from neuroendocrine cells
E
Gastric carcinoid tumor (instead of small intestinal) with mucosal venous drainage
Answer: B — Serotonin from EC-cell tumor; liver mets bypass hepatic MAO → systemic serotonin
Enterochromaffin (EC) cells in the small intestine normally secrete serotonin into portal venous drainage, where hepatic monoamine oxidase (MAO) metabolizes it to 5-HIAA before systemic circulation → no carcinoid symptoms. However, with liver metastases, tumor serotonin drains directly into systemic circulation, bypassing hepatic MAO degradation → high circulating serotonin → flushing, diarrhea, and carcinoid heart disease (serotonin causes endocardial fibrosis of right heart valves, notably tricuspid). The elevated 5-HIAA (urine accumulation) and chromogranin A confirm neuroendocrine tumor burden. Why other options fail: Histamine is a minor mediator in some carcinoids; VIPoma causes watery diarrhea, not carcinoid syndrome; norepinephrine doesn't cause classic carcinoid symptoms; gastric carcinoid has different vascular anatomy.
Question 10 · Dumping Syndrome Pathophysiology
A 62-year-old male s/p truncal vagotomy and pyloroplasty for peptic ulcer disease (10 years ago) now presents with postprandial symptoms: palpitations, diaphoresis, tremor, and reactive hypoglycemia 1 hour after eating. Blood glucose nadir is 58 mg/dL. His symptoms resolve when he eats frequent small meals and avoids simple sugars. Endoscopy rules out mechanical obstruction.
What is the primary mechanism of dumping syndrome in this patient?
A
Persistent vagal hyperactivity despite truncal vagotomy
B
Rapid gastric emptying → hyperosmotic load → fluid shift + reactive insulin surge → hypoglycemia
C
Gastrin deficiency causing impaired postprandial HCl secretion
D
Delayed gastric emptying with bacterial overgrowth producing gas
E
Pancreatic exocrine insufficiency mimicking dumping
Answer: B — Rapid gastric emptying → hyperosmotic load → fluid shift + reactive insulin surge → hypoglycemia
Truncal vagotomy disrupts the vagal signals that normally modulate antral contraction and pyloric relaxation → loss of rate control over gastric emptying. Food (especially simple sugars) empties rapidly into the duodenum → osmotic fluid shift from plasma into intestinal lumen (plasma osmolality drops) → intestinal distension triggers release of GLP-1 and other incretin hormones → exaggerated insulin secretion → rapid glucose clearance → reactive hypoglycemia (late dumping). Early dumping (first 15 min) causes flushing and palpitations from fluid shifts and catecholamine release. Pyloroplasty compounds this by eliminating pyloric resistance. Why other options fail: Vagotomy reduces, not increases, vagal tone; gastrin deficiency would cause hypersecretion (opposite); delayed emptying causes gastroparesis symptoms, not dumping; pancreatic insufficiency causes steatorrhea.
Question 11 · Leptin Resistance in Obesity
A 35-year-old female with severe obesity (BMI 42, weight 240 lbs at 5'4") presents for weight management. Lab work shows serum leptin 80 ng/mL (normal 4–6 ng/mL for lean individuals) and serum adiponectin is low. She reports constant hunger despite eating adequate calories. Genetic testing rules out leptin gene mutation (LEP). MRI hypothalamus shows normal pituitary anatomy. Fasting glucose and lipid panel are abnormal (insulin resistance, dyslipidemia).
How do you explain the obesity and constant hunger despite high leptin levels?
A
Congenital leptin deficiency (same presentation as genetic)
B
Leptin resistance — leptin receptors unresponsive in arcuate nucleus
C
Ghrelin hyperactivity overwhelming leptin signaling
D
POMC neuron mutation in the arcuate nucleus
E
Insulinoma with reactive obesity from constant hyperinsulinemia
Answer: B — Leptin resistance — leptin receptors unresponsive in arcuate nucleus
Leptin resistance is a hallmark of common obesity: adipose tissue produces abundant leptin, but hypothalamic leptin receptors (OB-R) in the arcuate nucleus become desensitized to leptin signaling (mechanisms: chronic inflammation, elevated triglycerides, ER stress, suppressor of cytokine signaling [SOCS-3] upregulation). Result: despite high circulating leptin, the brain perceives starvation → persistent hunger and reduced energy expenditure → weight gain. This explains the paradox of high leptin + obesity + hyperphagia, differentiating it from true leptin deficiency. Why other options fail: Genetic leptin deficiency would have low leptin (not high); ghrelin is elevated in weight loss, not obesity; POMC mutations cause early-onset obesity; insulinomas cause hypoglycemia, not primary obesity.
Question 12 · Salivary Composition & Osmolality
A student researcher collects saliva samples from healthy volunteers in a quiet resting state (low flow rate, ~0.5 mL/min). Compared to plasma, the low-flow saliva is analyzed for electrolyte composition and osmolality.
Which of the following best describes low-flow saliva composition relative to plasma?
A
Isotonic to plasma with similar Na⁺ and Cl⁻
B
Hypertonic with high Na⁺ and Cl⁻ concentrations
C
Hypotonic with low Na⁺ and Cl⁻, high K⁺ and HCO₃⁻
D
Low osmolality but with paradoxically high Na⁺ concentration
E
Variable osmolality depending on time of day
Answer: C — Hypotonic with low Na⁺ and Cl⁻, high K⁺ and HCO₃⁻
Salivary glands produce hypotonic saliva via a two-step process: (1) Acinar cells secrete isotonic primary saliva with plasma-like electrolytes; (2) duct cells actively reabsorb Na⁺ and Cl⁻ via ENaC and NCC transporters, while secreting K⁺ via ROMK channels and HCO₃⁻ via secretory pathways. At low flow rates, transit time through ducts is long → more reabsorption → maximally hypotonic saliva (osmolality ~50 mOsm/kg). At high flow rates (stimulated saliva), less time for reabsorption → more isotonic (~100 mOsm/kg). Why other options fail: Hypertonic saliva doesn't occur physiologically; osmolality is fixed by mechanism, not time of day.
Question 13 · Enterokinase & Trypsin Activation
A 5-month-old male infant presents with failure to thrive, chronic diarrhea, and protein malabsorption (low serum albumin, elevated fecal nitrogen). Pancreatic imaging is normal; pancreatic enzyme levels (amylase, lipase) are normal. Duodenal biopsy shows normal intestinal architecture. Stool collection shows markedly elevated trypsinogen (10-fold normal) but absent trypsin activity. Trypsin-like activity is restored when duodenal juice is treated with enterokinase in vitro.
What enzyme is missing in this patient?
A
Trypsin itself — pancreatic deficiency
B
Chymotrypsin — protein digestion failure
C
Enterokinase (enteropeptidase) — failure to activate trypsinogen
D
Cholecystokinin (CCK) — impaired pancreatic secretion
E
Pancreatic lipase — fat malabsorption only
Answer: C — Enterokinase (enteropeptidase) — failure to activate trypsinogen
Enterokinase is a serine protease secreted by duodenal mucosa that catalyzes the first step of pancreatic enzyme activation: cleavage of the N-terminal hexapeptide from trypsinogen → active trypsin. Trypsin then autoCatalytically activates more trypsinogen and activates other zymogens (chymotrypsinogen, proelastase, procarboxypeptidases). Enterokinase deficiency → accumulation of inactive trypsinogen → no trypsin available to activate downstream enzymes → protein malabsorption. Note: pancreatic enzyme levels are normal because the problem is not pancreatic synthesis, but duodenal activation. Why other options fail: Trypsin deficiency would show low trypsin despite high trypsinogen (impossible at origin); chymotrypsin deficiency wouldn't block trypsin generation; CCK deficiency impairs secretion of all enzymes; lipase deficiency affects fats, not proteins.
Question 14 · Achlorhydria & B12 Absorption Cascade
A 70-year-old male on long-term omeprazole (15 years) for GERD and s/p gastrectomy (30 years ago) for gastric ulcer now presents with macrocytic anemia (MCV 118 fL), low B12 (100 pg/mL), and elevated methylmalonic acid. Gastric pH is 7.1. Serum intrinsic factor and parietal cell antibodies are negative (no autoimmune basis). Upper endoscopy shows atrophic gastric mucosa.
What is the mechanism of B12 malabsorption in this patient?
A
Autoimmune destruction of intrinsic factor-producing cells
B
↓Acid → ↓pepsin → R-protein not cleaved from food B12 → no IF binding → no ileal absorption
C
Terminal ileal inflammation → cubilin receptor loss
D
Folate competition for ileal transporters reduces B12 uptake
E
Dietary B12 insufficiency with normal absorption physiology
Answer: B — ↓Acid → ↓pepsin → R-protein not cleaved from food B12 → no IF binding → no ileal absorption
Dietary B12 is bound to R-protein (cobalamin-binding protein) in food. In the stomach, HCl → pepsin → proteolysis → R-protein is cleaved, releasing B12 for binding to intrinsic factor (IF). Achlorhydria (from gastrectomy + long-term PPI) → no acid → impaired pepsin activity → R-protein remains bound to B12 → IF cannot displace it → B12-IF complex doesn't form → no cubilin-mediated absorption in terminal ileum. Notably, IF antibodies are negative (not pernicious anemia), and remaining gastric tissue (post-gastrectomy) still makes IF, but it has no free B12 substrate to bind. Why other options fail: Autoimmune disease excluded by negative antibodies; terminal ileal inflammation not present; folate uses different transporters (not competitive); diet alone is unlikely to cause such deficiency with supplements.
Question 15 · GLP-1 Receptor Agonists & Gastric Function
A 48-year-old female with type 2 diabetes mellitus is started on semaglutide (GLP-1 agonist) 0.5 mg weekly. Within 2 weeks, she reports nausea and marked appetite suppression. Her HbA1c drops from 8.5% to 7.2% within 3 months. She experiences weight loss (8 lbs) and reports feeling full after small meals. Fasting glucose normalizes, and postprandial glucose spikes are blunted.
Which of the following best describes the GLP-1 agonist's direct effect on gastric function?
A
↑Glucagon secretion stimulating hepatic glucose production
B
↓Gastric emptying + glucose-dependent insulin release + ↑satiety via CNS
C
↑Gastric acid secretion enhancing peptic digestion
D
↑Pancreatic lipase and amylase secretion
E
↑Bile acid secretion increasing lipid emulsification
Answer: B — ↓Gastric emptying + glucose-dependent insulin release + ↑satiety via CNS
GLP-1 is secreted by L cells in response to nutrient intake; GLP-1 receptors on vagal afferent neurons and brainstem trigger satiety signals, while receptors on gastric smooth muscle slow antral contractions → delayed gastric emptying. Slower nutrient delivery → more gradual glucose absorption → less postprandial hyperglycemia and less reactive hypoglycemia. GLP-1 also enhances glucose-dependent (not basal) insulin secretion → lower fasting glucose. The appetite suppression and delayed emptying contribute to weight loss. Why other options fail: GLP-1 suppresses glucagon (not stimulates, except in hypoglycemia); acid secretion is unchanged; pancreatic enzyme secretion is not a primary GLP-1 target; bile acid secretion is not affected.
Question 16 · Stress Ulcer Pathophysiology
A 60-year-old male admitted to ICU following a major burn injury (35% TBSA) develops gastric ulceration within 48 hours despite normal gastrin levels. Upper endoscopy shows acute gastric ulcers in the fundus (Curling ulcers). His hemodynamics are unstable with low blood pressure and elevated lactate. Gastric pH is low at 1.5. H. pylori testing is negative.
What is the primary mechanism of stress ulcer formation in this patient?
A
↑Gastrin from stress-induced G-cell hyperplasia
B
↓Mucosal blood flow + ↓mucus/HCO₃⁻ secretion → acid back-diffusion & tissue necrosis
C
H. pylori reactivation from immunosuppression
D
NSAID-induced COX-1 inhibition in the gastric mucosa
E
↑Vagal tone stimulating excessive acid secretion
Answer: B — ↓Mucosal blood flow + ↓mucus/HCO₃⁻ secretion → acid back-diffusion & tissue necrosis
Stress ulcers (Curling ulcers in burns, Cushing ulcers in CNS trauma) result from splanchnic vasoconstriction secondary to severe stress, sepsis, or shock → reduced mucosal perfusion → ↓production of protective mucus and HCO₃⁻ → acid penetrates damaged epithelium and damages submucosal vessels → ischemic necrosis. Acid levels are normal or slightly elevated, not excessive (excluding gastrinoma/severe acid hypersecretion). Prevention with PPIs or H₂-blockers works by reducing acid, not by improving perfusion. Why other options fail: Gastrin is typically normal in stress; H. pylori requires chronic colonization, not acute reactivation; NSAIDs not involved (no drug history); vagal tone doesn't explain ischemic mechanism.
Question 17 · Cholecystokinin & Postprandial Secretion
A patient with cholesterol gallstones experiences postprandial right upper quadrant pain 30 minutes after eating fatty meals. Ultrasound shows gallstone impaction. She is scheduled for laparoscopic cholecystectomy. You review her GI physiology with her.
Which hormone is primarily responsible for stimulating gallbladder contraction and pancreatic enzyme secretion in response to meals?
A
Secretin — stimulates HCO₃⁻-rich pancreatic juice
B
Gastrin — stimulates acid secretion and gastric motility
C
Cholecystokinin (CCK) from I cells → GB contraction + pancreatic enzyme secretion
D
Glucose-dependent insulinotropic peptide (GIP) — enhances insulin secretion
E
Motilin — initiates migrating motor complex during fasting
Answer: C — Cholecystokinin (CCK) from I cells → GB contraction + pancreatic enzyme secretion
CCK is secreted by I cells in the duodenum and jejunum in response to fatty acids and peptides in chyme. CCK acts on CCK-A receptors on gallbladder smooth muscle → contraction → bile release into duodenum (needed for fat emulsification and absorption). CCK also stimulates pancreatic acinar cells → release of digestive enzymes (amylase, lipase, proteases) into the duct system. In gallstone disease, GB contraction against an obstructed cystic duct → colicky pain. Why other options fail: Secretin stimulates bicarbonate secretion but not GB contraction (works synergistically with CCK); gastrin acts on stomach; GIP enhances insulin but doesn't contract GB; motilin controls fasting motor activity, not postprandial digestion.
Question 18 · Colonic Electrolyte Secretion & Acidosis
A 35-year-old male with chronic diarrhea (8–10 loose stools/day) of 2 years' duration presents with hypokalemia (K⁺ 2.9 mEq/L), non-anion gap metabolic acidosis (pH 7.28, HCO₃⁻ 16 mEq/L), and normal serum creatinine. Stool osmolal gap is 150 mOsm/kg (normal <50), indicating osmotic diarrhea. Colonoscopy shows patchy erythema and ulceration consistent with inflammatory bowel disease. Fecal K⁺ is markedly elevated.
In what segment of GI tract are K⁺ and HCO₃⁻ being lost in this patient?
A
Stomach — HCl loss causing alkalosis (not acidosis)
B
Small intestine only — limited K⁺ transport
C
Colon — aldosterone-driven K⁺ secretion + HCO₃⁻/Cl⁻ antiporter loss
D
Kidneys — renal wasting of K⁺ and HCO₃⁻
E
Pancreas — ductal secretion of enzymes and electrolytes
Answer: C — Colon — aldosterone-driven K⁺ secretion + HCO₃⁻/Cl⁻ antiporter loss
The colon is the primary site of K⁺ loss in diarrhea. Colonocytes have two key mechanisms: (1) Aldosterone-stimulated ROMK (renal outer medullary potassium) channels in the apical membrane actively secrete K⁺ into the lumen; (2) HCO₃⁻/Cl⁻ exchangers (DRA, SLC26A3) in beta-intercalated cells reabsorb HCO₃⁻ in exchange for Cl⁻ secretion. In diarrhea, rapid transit limits reabsorption. In IBD, inflamed colonic mucosa increases K⁺ secretion and impairs HCO₃⁻ recovery → both hypokalemia and metabolic acidosis (loss of HCO₃⁻-rich alkaline fluid). Why other options fail: Stomach losses cause metabolic alkalosis (HCl loss); small intestine is permeable but doesn't actively secrete K⁺; kidneys can conserve K⁺ if needed; pancreas doesn't lose these electrolytes in GI disease.