The commonest side-effect of Cisapride is –
**Core Concept**
Cisapride is a prokinetic agent that acts as a selective serotonin 5-HT4 receptor agonist, increasing gastrointestinal motility by enhancing acetylcholine release from the enteric nervous system. Its mechanism of action is distinct from other prokinetic agents like metoclopramide, which acts as a dopamine antagonist.
**Why the Correct Answer is Right**
The commonest side-effect of Cisapride is **gastroesophageal reflux disease (GERD)**. This occurs due to the increased lower esophageal sphincter (LES) pressure caused by cisapride's action on 5-HT4 receptors. However, this effect can paradoxically lead to increased reflux in some patients. Additionally, cisapride's enhancement of acetylcholine release can cause gastrointestinal side effects like diarrhea, abdominal pain, and flatulence.
**Why Each Wrong Option is Incorrect**
* **Option A:** Although cisapride can cause QT interval prolongation, it is not the most common side-effect. QT interval prolongation is a serious cardiac side effect that requires monitoring, but it is less common than GERD.
* **Option B:** Diarrhea is a possible side effect of cisapride, but it is not the most common. Diarrhea is more frequently associated with other medications that increase gastrointestinal motility, like metoclopramide.
* **Option C:** Hypersensitivity reactions, including anaphylaxis, are rare but serious side effects of cisapride. However, they are not the most common side effect.
**Clinical Pearl / High-Yield Fact**
Remember that cisapride's mechanism of action on 5-HT4 receptors can paradoxically increase GERD symptoms in some patients. This highlights the importance of monitoring patients on cisapride for GERD symptoms and adjusting treatment as necessary.
**Correct Answer: C. Gastroesophageal reflux disease (GERD)**