This antihistaminic drug can cause cardiac arrhythmia at high dose by blocking cardiac K + channels. It is most likely to be:
**Core Concept**
The question is testing the effect of antihistaminic drugs on cardiac potassium channels, specifically the risk of causing cardiac arrhythmias at high doses.
**Why the Correct Answer is Right**
The correct answer is a non-sedating antihistamine that is known to block cardiac potassium channels. This action can lead to an increase in the duration of the action potential, thereby prolonging the QT interval and increasing the risk of torsades de pointes, a specific type of cardiac arrhythmia. **Terfenadine** and **Astemizole** are examples of such antihistamines that have been withdrawn from the market due to this risk.
**Why Each Wrong Option is Incorrect**
* **Option A:** **Cetirizine** is a second-generation antihistamine that is primarily metabolized by the liver and does not have significant effects on cardiac potassium channels.
* **Option B:** **Fexofenadine** is another non-sedating antihistamine that is primarily eliminated through the kidneys and does not have significant effects on cardiac potassium channels.
* **Option C:** **Loratadine** is a non-sedating antihistamine that is primarily metabolized by the liver and does not have significant effects on cardiac potassium channels.
**Clinical Pearl / High-Yield Fact**
When prescribing antihistamines, especially at high doses, it is essential to be aware of their potential effects on cardiac potassium channels and to monitor patients closely for signs of cardiac arrhythmias.
**Correct Answer:** C.