**Core Concept**
Neostigmine is an anticholinesterase inhibitor used to improve muscle strength in patients with myasthenia gravis and to reverse the effects of non-depolarizing muscle relaxants. Its inability to cross the blood-brain barrier is due to its polar nature.

**Why the Correct Answer is Right**
Neostigmine's inability to cross the blood-brain barrier is primarily due to its charged quaternary ammonium group, which prevents it from diffusing through the lipid-rich cell membranes. This property makes neostigmine a useful drug for treating peripheral neuromuscular junction disorders without causing central nervous system (CNS) effects.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect as it does not specify the reason for neostigmine's inability to cross the blood-brain barrier.
* **Option B:** This option is incorrect as it does not accurately describe the chemical structure of neostigmine.
* **Option C:** This option is incorrect as it does not provide a valid reason for neostigmine's inability to cross the blood-brain barrier.

**Clinical Pearl / High-Yield Fact**
The blood-brain barrier is a highly selective semipermeable membrane that restricts the passage of charged ions, polar molecules, and large molecules into the CNS, making it an essential barrier for maintaining the homeostasis of the brain.

**Correct Answer:** C.