**Core Concept**
The question tests the student's knowledge of the pharmacological effects of **succinylcholine**, a depolarizing neuromuscular blocking agent, on the heart. **Succinylcholine** can cause changes in heart rhythm due to its effects on the autonomic nervous system and the heart itself.

**Why the Correct Answer is Right**
The most common arrhythmia associated with **succinylcholine** is **bradycardia**, particularly in children and young adults. This is because **succinylcholine** can stimulate both nicotinic and muscarinic receptors, leading to an increase in vagal tone, which slows the heart rate.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because while **tachycardia** can occur due to sympathetic stimulation, it is not the most common arrhythmia associated with **succinylcholine**.
**Option C:** Incorrect as **ventricular fibrillation** is a life-threatening arrhythmia not typically associated with **succinylcholine** administration.
**Option D:** Incorrect because **atrial fibrillation** is not commonly linked to **succinylcholine** use.

**Clinical Pearl / High-Yield Fact**
Remember, **bradycardia** is more common after a second dose of **succinylcholine**, likely due to the increased muscarinic effects.

**Correct Answer:** B. Bradycardia.