A 52-year-old man from Delhi presents with a 3-month history of epigastric pain, heartburn, and nausea. He reports taking NSAIDs regularly for chronic knee pain. Endoscopy reveals a 2 cm ulcer in the antrum with surrounding erythema. Histology is negative for H. pylori. A fasting serum gastrin level is 45 pg/mL (normal <100 pg/mL). Which of the following best explains the mechanism of ulcer formation in this patient?

Explanation

## Mechanism of NSAID-Induced Gastric Ulceration **Key Point:** NSAIDs cause gastric ulcers through two independent mechanisms: (1) direct mucosal injury via inhibition of COX-1, reducing protective prostaglandins (PGE₂, PGI₂), and (2) paradoxically increasing acid secretion through loss of prostaglandin-mediated inhibition of parietal cells. ### Pathophysiology of NSAID Ulcers NSAIDs inhibit cyclooxygenase enzymes, which are essential for: - Mucus and bicarbonate secretion - Mucosal blood flow - Epithelial cell proliferation and restitution - **Inhibition of gastric acid secretion** (prostaglandins normally suppress acid via G cells) When prostaglandins are depleted: - Parietal cell sensitivity to acid secretagogues (gastrin, histamine, ACh) increases - The antral mucosa loses its ability to suppress acid secretion - Net result: increased acid + decreased mucosal protection = ulcer **Clinical Pearl:** The normal gastrin level (45 pg/mL) rules out Zollinger-Ellison syndrome, confirming NSAID-induced injury as the primary mechanism rather than gastrin-driven hypersecretion. ### Why This Patient's Presentation Fits | Feature | Finding | Significance | |---------|---------|---------------| | NSAID use | Yes (chronic) | Primary risk factor | | H. pylori | Negative | Excludes infectious cause | | Gastrin level | Normal | Rules out ZES | | Antral location | Yes | Typical for NSAID ulcers | | Fasting acid | Not elevated | Acid secretion normal at baseline, but increased in response to stimuli | **High-Yield:** NSAID ulcers are acid-dependent but NOT acid-hypersecretory — the problem is loss of mucosal defense, not excess acid production. ## Regulation of Gastric Acid Secretion (Normal Physiology) ```mermaid flowchart TD A[Antral G cells]:::outcome -->|Gastrin| B[Parietal cells]:::action C[Enterochromaffin-like cells]:::outcome -->|Histamine| B D[Vagus nerve]:::outcome -->|ACh| B B -->|Acid secretion| E[HCl into lumen]:::action F[Somatostatin from D cells]:::outcome -->|Inhibits| A F -->|Inhibits| C G[Prostaglandins]:::outcome -->|Enhance| F G -->|Suppress acid| B H[NSAID use]:::urgent -->|Blocks PG synthesis| G H -->|Direct mucosal injury| I[Ulcer formation]:::urgent ``` **Mnemonic: GAH** — **G**astrin, **A**cetylcholine, **H**istamine are the three main acid secretagogues acting on parietal cells. [cite:Guyton & Hall Textbook of Medical Physiology Ch 64]