Which of the following does not cross the blood-brain barrier?
**Core Concept**
The blood-brain barrier (BBB) is a highly selective permeable barrier that separates the circulating blood from the brain's extracellular fluid in the central nervous system (CNS). It is composed of endothelial cells, astrocytes, and pericytes, which work together to restrict the passage of certain molecules, ions, and substances into the brain.
**Why the Correct Answer is Right**
The BBB is relatively impermeable to hydrophilic (water-soluble) substances, such as ions and polar molecules. However, certain lipophilic (fat-soluble) substances, including some anesthetics, steroids, and lipophilic medications, can diffuse across the BBB. The correct answer is a substance that is hydrophilic and cannot cross the BBB.
**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it is a lipophilic substance that can diffuse across the BBB due to its high lipid solubility.
**Option B:** Incorrect because it is a small, lipophilic molecule that can cross the BBB.
**Option C:** Incorrect because it is a peptide hormone that can cross the BBB through specific transport mechanisms.
**Clinical Pearl / High-Yield Fact**
Substances that cannot cross the BBB, such as certain antibiotics and antiviral medications, require alternative routes of administration, such as intrathecal or intraventricular injection, to achieve therapeutic concentrations in the CNS.
**Correct Answer: A. Water**