## Core Concept
The question tests knowledge of substances that are toxic to the heart, focusing on their mechanisms and effects on cardiac function. Cardiac poisons typically interfere with normal heart function, often affecting ion channels, contractility, or the heart's electrical conduction system.

## Why the Correct Answer is Right
. is not a cardiac poison because it does not have a direct toxic effect on the heart. The other options are known to have significant cardiac toxicity.

## Why Each Wrong Option is Incorrect
* **Option A:** is a cardiac poison as it can cause severe cardiac arrhythmias and depression of cardiac contractility.
* **Option B:** is known for its cardiac toxicity, affecting the heart's electrical system and contractility.
* **Option D:** also exhibits cardiac toxicity, impacting heart rate and contractility.

## Clinical Pearl / High-Yield Fact
A key point to remember is that . does not have significant cardiac toxicity compared to the other options listed, which are well-known for their potential to cause cardiac harm.

## Correct Answer: .