Which best describes the mechanism of action of proton pump inhibitors and a common adverse effect?

• A. Inhibit the proton pump in gastric parietal cells; adverse effects include C. difficile risk and malabsorption (e.g., B12).
• B. Block histamine H2 receptors.
• C. Neutralize stomach acid directly in lumen.
• D. Inhibit COX-2 to reduce inflammation.

Answer: (A)

Proton pump inhibitors work by blocking the final step of acid production in the stomach: the proton pump (H+/K+-ATPase) on gastric parietal cells. They are prodrugs that become active in the acidic environment and form an irreversible bond with the pump, shutting down acid secretion. Because the binding is irreversible, acid production stays low until new pumps are made, which means the effect lasts beyond the dosing period and explains why relief can persist even after stopping the medicine.

A common adverse effect linked to this mechanism is an increased risk of infections like C. difficile, since less stomach acid makes it easier for some organisms to survive and proliferate. Another consequence is malabsorption of nutrients that depend on stomach acidity for proper release and absorption, such as vitamin B12.

The other options describe different drugs or actions: blocking H2 receptors targets a different part of the acid-secretion pathway; neutralizing acid in the lumen is done by antacids; and inhibiting COX-2 relates to anti-inflammatory effects, not acid suppression.