Which of the following is a proton pump inhibitor (PPI)?

• A. Metformin
• B. Lisinopril
• C. Acetaminophen
• D. Omeprazole

Answer: (D)

Proton pump inhibitors reduce gastric acid by blocking the final step of acid production in the stomach, the H+/K+ ATPase enzyme on parietal cells. By inhibiting this pump, they prevent hydrogen ions from being pumped into the stomach lumen, giving the most potent and sustained suppression of acid. Omeprazole is a classic example of this class—when it reaches the acidic environment of the parietal cell canaliculi, it is activated and forms a stable block on the proton pump, leading to a significant drop in acid output. Because the pump activity increases with meals, PPIs are typically given about 30 to 60 minutes before eating to maximize their effect. This makes omeprazole the correct choice for a proton pump inhibitor among the options. The other drugs listed serve different roles: metformin is used for type 2 diabetes, lisinopril is an antihypertensive/ACE inhibitor, and acetaminophen is an analgesic/antipyretic, none of which act as proton pump inhibitors.