A 52-year-old man from Delhi presents with a 6-month history of epigastric pain, heartburn, and nausea. Upper endoscopy reveals a duodenal ulcer with active bleeding. Serum gastrin level is 850 pg/mL (normal <100). Gastric pH is 1.5. On further questioning, he reports recurrent diarrhoea and a family history of pancreatic neuroendocrine tumours. Which of the following mechanisms best explains the excessive gastric acid secretion in this patient?

Explanation

## Diagnosis: Zollinger–Ellison Syndrome (ZES) ### Clinical Presentation This patient has classic features of ZES: - Severe, refractory peptic ulcer disease with complications (bleeding) - Markedly elevated fasting serum gastrin (850 pg/mL, normal <100) - Low gastric pH (1.5) indicating excessive acid production - Chronic diarrhoea (due to acid inactivation of pancreatic enzymes and mucosal damage) - Family history of pancreatic neuroendocrine tumours (suggests MEN-1 syndrome with gastrinoma) ### Mechanism of Acid Hypersecretion **Key Point:** In ZES, a gastrin-secreting neuroendocrine tumour (gastrinoma) produces unregulated gastrin, which continuously stimulates parietal cells to secrete acid at maximal rates, bypassing normal feedback inhibition. ### Normal Gastric Acid Regulation | Phase | Stimulus | Mediator | Effect on Acid | |-------|----------|----------|----------------| | **Cephalic** | Sight, smell, taste | Vagal ACh → M3 receptors | ↑ Acid | | **Gastric** | Protein, distension | Gastrin (G cells) | ↑ Acid | | **Intestinal** | Acid, fat, hyperosmolar chyme | Secretin, CCK, GIP | ↓ Acid | | **Feedback** | Low pH (<3) | Somatostatin (D cells) | ↓ Gastrin release | **High-Yield:** Normal gastrin-mediated acid secretion is self-limiting because: 1. Acid stimulates D cells to release somatostatin 2. Somatostatin inhibits further gastrin release from G cells 3. This negative feedback maintains pH 3–5 in the antrum In ZES, the tumour secretes gastrin **independently** of pH feedback, so: - Serum gastrin remains elevated even at pH <2 - Parietal cells are continuously maximally stimulated - Somatostatin feedback is overwhelmed and ineffective - Acid output reaches 20–30 mEq/h (normal fasting <5 mEq/h) **Clinical Pearl:** The diagnostic hallmark of ZES is a fasting serum gastrin >1000 pg/mL with gastric pH <2, or gastrin 100–1000 pg/mL with pH <2 and a positive secretin stimulation test (paradoxical rise in gastrin >200 pg/mL above baseline). ### Why This Mechanism Explains the Findings - **Elevated serum gastrin:** Autonomous tumour secretion - **Low pH:** Continuous parietal cell stimulation by gastrin - **Refractory ulcers:** Acid output exceeds mucosal defence capacity - **Diarrhoea:** Acid inactivates pancreatic lipase and damages small bowel mucosa [cite:Harrison 21e Ch 297]