**Core Concept**
Muscle relaxants are a class of medications used to facilitate intubation, reduce muscle spasms, and provide skeletal muscle relaxation. The elimination of these drugs from the body is crucial in determining their duration of action and potential side effects.

**Why the Correct Answer is Right**
The muscle relaxant excreted exclusively by the kidney is Vecuronium. Vecuronium is a non-depolarizing neuromuscular blocker that works by competitively blocking the nicotinic acetylcholine receptors at the neuromuscular junction. It is primarily eliminated through the kidneys, with a small amount metabolized by the liver. The elimination of vecuronium is primarily dependent on renal function, making it a suitable choice for patients with normal renal function.

**Why Each Wrong Option is Incorrect**
* **Option A:** Pancuronium is also a non-depolarizing neuromuscular blocker, but it is primarily eliminated through the liver and to a lesser extent through the kidneys.
* **Option B:** Atracurium is a non-depolarizing neuromuscular blocker that undergoes Hoffman elimination, a spontaneous degradation process that is not dependent on renal function.
* **Option C:** Succinylcholine is a depolarizing neuromuscular blocker that is primarily eliminated through the liver and to a lesser extent through the kidneys.

**Clinical Pearl / High-Yield Fact**
Vecuronium is a suitable choice for patients with normal renal function, but its elimination is significantly affected in patients with renal impairment. It is essential to adjust the dosage and monitor renal function in patients with impaired renal function to avoid prolonged muscle relaxation and potential toxicity.

**Correct Answer:** D. Vecuronium.