Which of the following is a commonly used proton pump inhibitor?

Omeprazole is a commonly used proton pump inhibitor (PPI) that works by irreversibly inhibiting the gastric proton pump, specifically the H+/K+ ATPase enzyme located in the gastric parietal cells. This action effectively reduces the production of stomach acid, making PPIs like omeprazole particularly useful in treating conditions such as gastroesophageal reflux disease (GERD), peptic ulcers, and Zollinger-Ellison syndrome.

In contrast, the other options represent different classes of medications. Calcium carbonate is an antacid that neutralizes stomach acid but does not inhibit its production. Ranitidine belongs to a class of drugs called H2 receptor antagonists, which reduce stomach acid by blocking histamine receptors, but this mechanism is distinct from that of PPIs. Aluminum hydroxide is also an antacid, similar to calcium carbonate, providing relief by neutralizing existing acid rather than reducing its secretion. These distinctions highlight omeprazole's unique role as a proton pump inhibitor among the listed options.