**Core Concept**
The question requires the identification of a muscle relaxant that is safe to use in patients with renal failure. This involves understanding the pharmacokinetics and metabolism of muscle relaxants, particularly how they are excreted from the body.

**Why the Correct Answer is Right**
The correct answer is Rocuronium, as it is primarily metabolized by the liver and does not require renal excretion for its elimination. This makes it a suitable choice for patients with renal failure. Rocuronium is a non-depolarizing neuromuscular blocker that acts by competitively blocking the nicotinic acetylcholine receptors at the neuromuscular junction.

**Why Each Wrong Option is Incorrect**
**Option A:** Vecuronium - Although vecuronium is also primarily metabolized by the liver, it is excreted in bile and can accumulate in patients with liver dysfunction, making it less ideal in renal failure.
**Option B:** Pancuronium - Pancuronium is excreted primarily through the kidneys, making it contraindicated in patients with renal failure.
**Option C:** Cisatracurium - Cisatracurium is metabolized by Hofmann elimination, a chemical process that is independent of renal or hepatic function, but it is still excreted in urine and may accumulate in patients with renal failure.

**Clinical Pearl / High-Yield Fact**
In patients with renal failure, it is essential to choose muscle relaxants that are primarily metabolized by the liver and do not rely on renal excretion for elimination.

**Correct Answer: C. Cisatracurium**