## Core Concept
Muscle contraction is primarily caused by the interaction between **acetylcholine (ACh)**, a neurotransmitter released by motor neurons, and **nicotinic acetylcholine receptors** on the muscle fiber membrane. This interaction leads to depolarization of the muscle fiber and subsequent contraction.

## Why the Correct Answer is Right
The correct answer involves the release of **acetylcholine (ACh)** from the terminal end of a motor neuron, which then binds to **nicotinic acetylcholine receptors** on the muscle fiber. This binding causes a rapid depolarization of the muscle fiber membrane, known as an **end-plate potential**, which can initiate an **action potential** in the muscle fiber. The action potential travels along the muscle fiber and leads to muscle contraction through the **sliding filament theory**, involving **actin** and **myosin** filaments.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because it does not accurately describe a mechanism by which muscle contraction is caused.
- **Option B:** This option is incorrect because it does not accurately describe a mechanism by which muscle contraction is caused.
- **Option D:** This option is incorrect because it does not accurately describe a mechanism by which muscle contraction is caused.

## Clinical Pearl / High-Yield Fact
A key point to remember is that **myasthenia gravis**, an autoimmune disease characterized by weakness and rapid fatigue of the muscles under voluntary control, is caused by antibodies blocking or destroying **nicotinic acetylcholine receptors** at the junction between the nerve and muscle. This results in decreased effectiveness of acetylcholine in stimulating muscle contraction.

## Correct Answer: C.