**Core Concept**
Vecuronium is a non-depolarizing skeletal muscle relaxant that competes with acetylcholine for binding sites at the neuromuscular junction, thereby inhibiting muscle contraction. It functions by blocking the action of acetylcholine at the nicotinic receptors on the muscle end-plate.

**Why the Correct Answer is Right**
Vecuronium acts by competitively inhibiting the binding of acetylcholine to the nicotinic receptors at the neuromuscular junction. This inhibition leads to a decrease in the depolarization of the muscle end-plate, resulting in muscle relaxation. The nicotinic receptors are a type of acetylcholine receptor that is responsible for transmitting signals from nerve terminals to muscle fibers. Vecuronium's action at these receptors is reversible, allowing for recovery of muscle function after the drug is eliminated.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because vecuronium does not act on muscarinic receptors, which are involved in parasympathetic transmission and are responsible for the effects of acetylcholine on smooth muscle, glands, and the heart.
**Option B:** Incorrect because vecuronium does not act on GABA receptors, which are involved in inhibitory neurotransmission in the central nervous system.
**Option C:** Incorrect because vecuronium does not act on NMDA receptors, which are involved in excitatory neurotransmission in the central nervous system.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that non-depolarizing muscle relaxants like vecuronium can be reversed with anticholinesterases, such as neostigmine, which increases acetylcholine levels at the neuromuscular junction and counteracts the effects of the muscle relaxant.

**Correct Answer: D. nicotinic receptors**