## **Core Concept**
The blood-brain barrier (BBB) is a highly selective semipermeable border 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 cross.

## **Why the Correct Answer is Right**
Among the given options, **b) Propranolol** and **d) Physostigmine** are known to cross the blood-brain barrier. However, the correct answer provided is not directly listed, suggesting a need to identify which of these can cross the BBB. **Propranolol**, a beta-blocker, is lipid-soluble and can cross the BBB. **Physostigmine**, an anticholinesterase, is also known to cross the BBB. Between these, **Physostigmine** is notably recognized for its ability to cross the BBB, making it effective in treating conditions like anticholinergic toxicity that affects the CNS.

## **Why Each Wrong Option is Incorrect**
- **Option A: Dopamine** - Dopamine itself does not effectively cross the blood-brain barrier; however, its precursors (like L-DOPA) can.
- **Option C: Glycopyrrolate** - This is a quaternary amine anticholinergic drug that does not cross the blood-brain barrier due to its charged nature and low lipid solubility.
- **Option E: Streptomycin** - This is an aminoglycoside antibiotic that does not cross the blood-brain barrier, primarily due to its polar, charged nature.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that lipophilic (lipid-soluble) drugs are more likely to cross the blood-brain barrier. This is why **Physostigmine**, despite being a somewhat polar molecule, can cross, but **Glycopyrrolate** and **Streptomycin**, being more polar and less lipid-soluble, cannot.

## **Correct Answer: D. Physostigmine**