**Core Concept**
Torsades de pointes is a specific type of abnormal heart rhythm that can be caused by medications that prolong the QT interval. This is a critical concern in patients taking medications that can interact with the cytochrome P450 enzyme system, particularly those that inhibit the activity of CYP3A4.

**Why the Correct Answer is Right**
Cisapride, a prokinetic agent used to treat GERD, is a potent inhibitor of the CYP3A4 enzyme. By inhibiting this enzyme, cisapride increases the levels of other medications, including erythromycin, which can also prolong the QT interval. The combination of cisapride and erythromycin significantly increases the risk of torsades de pointes, a potentially life-threatening arrhythmia.

**Why Each Wrong Option is Incorrect**
* **Option B:** Famotidine is a histamine H2-receptor antagonist used to treat GERD. It does not have significant effects on the QT interval or the CYP3A4 enzyme system.
* **Option C:** Lansoprazole is a proton pump inhibitor (PPI) used to treat GERD. While PPIs can interact with other medications, they are not typically associated with QT interval prolongation.
* **Option D:** Metoclopramide is a dopamine antagonist used to treat GERD and other gastrointestinal disorders. While it can have proarrhythmic effects, it is not typically associated with QT interval prolongation in the same way as cisapride.

**Clinical Pearl / High-Yield Fact**
When treating patients with multiple medications, it's essential to consider potential drug-drug interactions, particularly those involving the CYP3A4 enzyme system. This can help prevent serious adverse effects, such as torsades de pointes, which can be life-threatening.

**✓ Correct Answer: A. Cisapride**