A patient with gastroesophageal reflux disease (GERD) is prescribed the histamine antagonist, cimetidine. How does cimetidine pharmacologically reduce the amount of stomach acid?

A. By neutralizing existing stomach acid through its alkaline properties: Neutralizing existing acid is the mechanism of antacids, not H2 receptor antagonists. Cimetidine does not chemically neutralize acid; it reduces acid production. B. By stimulating the production of bicarbonate to buffer the stomach acid: Bicarbonate secretion is a natural protective mechanism of the gastric mucosa, but cimetidine does not stimulate bicarbonate. This does not explain its acid-reducing effect. C. By inhibiting the proton pump enzyme, preventing hydrogen ion exchange in parietal cells: Proton pump inhibitors (e.g., omeprazole) act on the H⁺/K⁺ ATPase enzyme in parietal cells. Cimetidine works through H2 receptor antagonism, not by directly inhibiting the proton pump. D. By selectively blocking H2 receptors in the stomach which suppresses gastric acid secretion: Cimetidine binds to H2 receptors on gastric parietal cells, preventing histamine from stimulating acid production. This decreases both basal and meal-stimulated gastric acid secretion, effectively reducing GERD symptoms.