All these EXCEPT one cross the blood-brain barrier:
**Core Concept**
The blood-brain barrier (BBB) is a highly selective permeable barrier formed by endothelial cells of the brain capillaries, which restricts the passage of substances from the bloodstream into the brain. The BBB is composed of tight junctions, restricted paracellular transport, and limited transcellular transport, making it a crucial protective mechanism against various pathogens and toxins.
**Why the Correct Answer is Right**
Substances that are non-polar, lipid-soluble, and small in size can easily cross the BBB through passive diffusion, including oxygen, carbon dioxide, and certain lipophilic drugs like steroids and certain anesthetics. However, the BBB restricts the passage of polar, water-soluble molecules, which are often eliminated through active efflux transporters like P-glycoprotein. The BBB also restricts the passage of certain ions, such as sodium, potassium, and chloride, due to the presence of ion pumps and channels.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it is a polar, water-soluble molecule, making it unable to cross the BBB through passive diffusion.
* **Option B:** This option is incorrect because it is a large, hydrophilic molecule, which is restricted by the tight junctions and limited transcellular transport of the BBB.
* **Option C:** This option is incorrect because it is a charged molecule, which is restricted by the ion pumps and channels of the BBB.
**Clinical Pearl / High-Yield Fact**
Substances that cross the BBB can have significant effects on the brain, including altered neurotransmitter levels, changes in neuronal excitability, and even neurotoxicity. Understanding the properties of substances that cross the BBB is crucial for the effective treatment of neurological disorders and the safe administration of medications.
**Correct Answer:** A.