**Core Concept**
H2 blockers, also known as histamine H2-receptor antagonists, are a class of medications that inhibit the action of histamine at the H2 receptors in the stomach lining, thereby reducing gastric acid secretion. This class of drugs is often used to treat conditions such as gastroesophageal reflux disease (GERD), peptic ulcers, and Zollinger-Ellison syndrome.

**Why the Correct Answer is Right**
The correct answer is the one that does not belong to the H2 blocker class. H2 blockers work by blocking the H2 receptors in the stomach lining, which are responsible for stimulating gastric acid secretion. This action reduces the amount of acid produced in the stomach, providing relief from symptoms such as heartburn and ulcers.

**Why Each Wrong Option is Incorrect**
* **Option A:** Ranitidine is a classic example of an H2 blocker, used to treat conditions such as GERD and ulcers. It works by blocking the H2 receptors in the stomach lining, thereby reducing gastric acid secretion.
* **Option B:** Famotidine is another H2 blocker, commonly used to treat conditions such as GERD and ulcers. It has a similar mechanism of action to ranitidine, blocking the H2 receptors in the stomach lining.
* **Option C:** Nizatidine is also an H2 blocker, used to treat conditions such as GERD and ulcers. It has a similar mechanism of action to ranitidine and famotidine, blocking the H2 receptors in the stomach lining.

**Clinical Pearl / High-Yield Fact**
The H2 blocker class of medications is often used in combination with antacids or proton pump inhibitors (PPIs) to provide additional relief from symptoms such as heartburn and ulcers.

**Correct Answer: C. Nizatidine is an H2 blocker, however, it is less commonly known.