Upon preventing acetylcholine from diffusing across a neuromuscular junction, which of the following will not occur?
A. No action potential will be produced in the affected muscle fibers.
B. The endoplasmic reticulum releases calcium ions (Ca²⁺⁺) into the cytoplasm.
C. Myosin will not bind to actin in the affected muscle fiber.
D. The affected muscle fiber will fail to contract.

Hint:
The chemical messenger in the body is known as a neurotransmitter. Their main function is to transmit signals from nerve cells to target cells. Muscles, glands, etc are included in the target cell. Acetylcholine, the first neurotransmitter discovered, was originally described as "vagus stuff" by Otto Loewi because of its ability to mimic the electrical stimulation of the vagus nerve.

Complete answer:
On the postsynaptic membrane, the acetylcholine binds to the nicotinic receptors. Because of this binding, the nicotinic receptor channel remains open and allows sodium ions enter to the muscle fiber. If enough of these sodium ions enter the muscle fibers to raise from their resting potential, they trigger a muscle action potential that spread across the fiber.
So, if the release of acetylcholine is prevented the release of sodium ions and hence action potential will not occur in the affected muscle fibers.
Hence the correct answer is No action potential will be produced in the affected muscle fibers.

Option ‘A’ is correct

Note:
Acetylcholine is known to be a neurotransmitter in all autonomic ganglia, at many autonomically innervated organs, at the neuromuscular junction, and at many synapses in the CNS. A neuromuscular junction is a synaptic connection between the terminal end of a motor nerve and a muscle.