**Core Concept**
Non-depolarizing muscle relaxants (NDMRs) are a class of medications used to induce muscle relaxation during surgical procedures. They work by competitively inhibiting the action of acetylcholine at the nicotinic acetylcholine receptors (nAChRs) on the motor endplate, preventing muscle contraction.

**Why the Correct Answer is Right**
The shortest acting non-depolarizing muscle relaxant is Mivacurium. Mivacurium has a rapid onset and short duration of action due to its ester linkage, which is hydrolyzed by plasma cholinesterase. This results in a shorter duration of action compared to other NDMRs like Vecuronium or Rocuronium. The rapid onset and short duration of action of Mivacurium make it suitable for procedures requiring brief periods of muscle relaxation.

**Why Each Wrong Option is Incorrect**
**Option A:** Vecuronium is a long-acting NDMR, commonly used in surgical procedures requiring prolonged muscle relaxation.
**Option B:** Rocuronium has a rapid onset, but its duration of action is longer than Mivacurium, making it less suitable for short procedures.
**Option C:** Atracurium is an intermediate-acting NDMR, with a duration of action that is longer than Mivacurium.

**Clinical Pearl / High-Yield Fact**
Mivacurium is metabolized by plasma cholinesterase, which can be affected by genetic variations or concurrent administration of certain medications. This can result in prolonged duration of action or increased sensitivity to the medication.

**Correct Answer: D. Mivacurium**