**Core Concept**
The neuromuscular junction (NMJ) is a synapse between a neuron and a muscle fiber, where acetylcholine (ACh) is released from the presynaptic neuron and binds to nicotinic acetylcholine receptors (nAChRs) on the postsynaptic muscle fiber, causing muscle contraction. Neuromuscular blockade occurs when the transmission of this signal is inhibited, typically by non-depolarizing or depolarizing muscle relaxants.

**Why the Correct Answer is Right**
The correct answer is the one that does not increase neuromuscular blockade. To understand this, let's consider how each of the options affects neuromuscular transmission:

* **Option A:** This option is a non-depolarizing muscle relaxant, such as rocuronium or vecuronium. These drugs compete with ACh for binding sites on the nAChRs, reducing the amount of ACh that can bind and thereby decreasing muscle contraction and increasing neuromuscular blockade.
* **Option B:** This option is a depolarizing muscle relaxant, such as succinylcholine. These drugs mimic the action of ACh, causing sustained depolarization of the muscle fiber and initial contraction, followed by prolonged blockage of neuromuscular transmission.
* **Option C:** This option is a calcium channel blocker, such as verapamil. Calcium is essential for the release of ACh from the presynaptic neuron and for muscle contraction. By blocking calcium channels, verapamil can decrease neuromuscular transmission and increase neuromuscular blockade.

**Why Each Wrong Option is Incorrect**
* **Option A:** This non-depolarizing muscle relaxant increases neuromuscular blockade by competing with ACh for binding sites on the nAChRs.
* **Option B:** This depolarizing muscle relaxant increases neuromuscular blockade by mimicking the action of ACh and causing sustained depolarization of the muscle fiber.
* **Option C:** This calcium channel blocker increases neuromuscular blockade by decreasing the release of ACh from the presynaptic neuron and reducing muscle contraction.

**Clinical Pearl / High-Yield Fact**
When administering muscle relaxants, it's essential to be aware of the potential for increased neuromuscular blockade, particularly in patients with underlying neuromuscular disease or those taking medications that interact with neuromuscular transmission.

**Correct Answer:** D.