**Core Concept**
Post-anaesthetic myalgia (PAM) is a type of muscle pain that occurs after general anaesthesia. It is a common side effect of certain anaesthetic agents, particularly those that affect the neuromuscular junction. The underlying mechanism involves the inhibition of acetylcholine release or its action at the neuromuscular junction, leading to muscle fasciculations and subsequent pain.

**Why the Correct Answer is Right**
The correct answer is **Succinylcholine**, a depolarizing neuromuscular blocking agent. Succinylcholine works by mimicking the action of acetylcholine at the neuromuscular junction, causing muscle depolarization and subsequent paralysis. However, this action also leads to muscle fasciculations, which can cause pain and discomfort. The muscle fasciculations are thought to be due to the rapid depolarization and subsequent repolarization of muscle fibers, leading to muscle cramping and pain.

**Why Each Wrong Option is Incorrect**
**Option A:** Vecuronium is a non-depolarizing neuromuscular blocking agent that does not cause muscle fasciculations and is less likely to cause post-anaesthetic myalgia.
**Option B:** Fentanyl is an opioid analgesic that is commonly used for pain relief during and after surgery, but it is not associated with post-anaesthetic myalgia.
**Option C:** Isoflurane is a volatile anaesthetic agent that can cause muscle relaxation and is not typically associated with post-anaesthetic myalgia.

**Clinical Pearl / High-Yield Fact**
Succinylcholine is a depolarizing neuromuscular blocking agent that should be used with caution in patients with a history of muscle disease, particularly those with myasthenia gravis, as it can exacerbate muscle weakness.

**Correct Answer:** C. Succinylcholine