**Core Concept**
Neostigmine is a cholinesterase inhibitor used in the treatment of Myasthenia gravis, a chronic autoimmune disorder characterized by muscle weakness and fatigue. The underlying principle being tested is the mechanism of acetylcholine (ACh) regulation in the neuromuscular junction.

**Why the Correct Answer is Right**
Neostigmine acts by inhibiting the enzyme acetylcholinesterase (AChE), which breaks down acetylcholine into choline and acetate. By preventing the degradation of ACh, neostigmine increases the concentration of ACh in the synaptic cleft, thereby enhancing its action on nicotinic receptors at the neuromuscular junction. This leads to improved muscle contraction and strength in patients with Myasthenia gravis.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because neostigmine does not act by directly stimulating nicotinic receptors. While it does increase ACh levels, it does so by inhibiting AChE, not by directly activating the receptors.
**Option B:** This option is incorrect because neostigmine does not act by inhibiting the release of acetylcholine from the terminal end of the motor neuron. Its primary mechanism is through the inhibition of AChE.
**Option C:** This option is incorrect because neostigmine does not act by increasing the synthesis of acetylcholine. Its primary mechanism is through the inhibition of AChE, which prevents the breakdown of ACh.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that neostigmine's effects are reversible, meaning that its action is terminated once it is metabolized or excreted. This is in contrast to irreversible AChE inhibitors, which can have more prolonged effects.

**Correct Answer: C. Increasing the concentration of acetylcholine in the synaptic cleft by inhibiting acetylcholinesterase.**