Which of the following is NOT considered a proton pump inhibitor?

Ranitidine (Zantac) is not classified as a proton pump inhibitor (PPI). Instead, it falls under the category of H2 receptor antagonists, which work by blocking histamine H2 receptors in the stomach lining, thereby reducing the production of gastric acid. Proton pump inhibitors, such as omeprazole, esomeprazole, and pantoprazole, function by directly inhibiting the hydrogen/potassium ATPase enzyme system (the proton pump), leading to a more effective and lasting reduction in stomach acid secretion compared to H2 antagonists. This mechanism of action differentiates PPIs from H2 blockers like ranitidine, making ranitidine the correct choice for the question regarding which option is not a proton pump inhibitor.