Which of the following is a common example of an H2 receptor antagonist?

Ranitidine is a well-known example of an H2 receptor antagonist, widely used in both human and veterinary medicine. H2 receptor antagonists work by blocking histamine from binding to H2 receptors in the stomach lining, which reduces the production of gastric acid. This mechanism is particularly beneficial in treating conditions such as gastric ulcers and gastroesophageal reflux disease, where excess stomach acid is problematic.

In the context of veterinary pharmacology, understanding the role of H2 receptor antagonists like ranitidine is crucial for managing gastrointestinal issues in animals. They can help prevent stress-induced ulceration and provide symptomatic relief from acid-related disorders. While other drugs listed, such as amoxicillin and metronidazole, are used for different therapeutic purposes, they do not function as H2 receptor antagonists. Amoxicillin is an antibiotic, metronidazole is often used for its antibacterial and antiprotozoal properties, and phenobarbital is a barbiturate primarily used as an anticonvulsant. Therefore, the distinguishing characteristic of ranitidine as an H2 receptor antagonist clarifies why it is the correct answer among the provided options.