## Core Concept
The blood-brain barrier (BBB) is a highly selective permeable barrier that separates the circulating blood from the brain and extracellular fluid in the central nervous system (CNS). It is composed of endothelial cells that line the brain's capillaries, which are tightly joined together by tight junctions. This barrier restricts the passage of solutes, cells, and proteins into the CNS, allowing only specific substances to pass through.

## Why the Correct Answer is Right
Quaternary ammonium compounds, such as **atropine** and **tubocurarine**, are known for their inability to cross the blood-brain barrier due to their large size and positive charge. This characteristic limits their action to the peripheral nervous system.

## Why Each Wrong Option is Incorrect
* **Option A:** - This option is not provided, but generally, lipophilic substances like **alcohol** and certain anesthetics can cross the BBB.
* **Option B:** - This option is not provided, but substances like **morphine** can cross the BBB.
* **Option D:** - This option is not provided, but certain drugs like **amphetamines** can cross the BBB.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the blood-brain barrier can be circumvented by certain drugs or conditions. For instance, **pyrogens** can induce fever by acting on the hypothalamus, and certain drugs like **oseltamivir** can act on the brain. However, quaternary ammonium compounds are notable for their inability to cross the BBB.

## Correct Answer: D. Atropine does cross but among given option quaternary Ammonium compounds like D. Tubocurarine.