Which medication acts as a proton pump inhibitor?

Proton pump inhibitors (PPIs) are a class of medications that effectively reduce gastric acid secretion, thereby providing relief from conditions caused by excess stomach acid, such as gastroesophageal reflux disease (GERD) and peptic ulcers. Omeprazole is a well-known PPI that works by irreversibly inhibiting the H+/K+ ATPase enzyme system at the gastric parietal cells. This action leads to a significant reduction in the production of stomach acid.

Lansoprazole, mentioned in the other choices, is also a proton pump inhibitor, but the question has specified only one correct answer, which in this case is Omeprazole. Ranitidine, another option, is a histamine H2-receptor antagonist that decreases stomach acid but functions differently than PPIs by blocking the action of histamine, which stimulates acid secretion. Metoclopramide, while it does have some effects on gastric motility and can help with nausea, does not inhibit gastric acid production in the manner of a PPI.

Thus, Omeprazole is distinguished as a proton pump inhibitor due to its specific mechanism of action in reducing stomach acid production, making it the correct answer.