**Core Concept**
The blood–brain barrier (BBB) is a highly selective permeable barrier that restricts the passage of substances from the bloodstream into the brain. Only a few drugs are able to cross this barrier due to their lipophilicity, molecular size, and charge. The ability of a drug to cross the BBB is crucial for its efficacy in treating central nervous system (CNS) disorders.

**Why the Correct Answer is Right**
The correct answer is the drug that does not have the necessary properties to cross the BBB. Most drugs that are lipophilic and small in size can cross the BBB, but some cannot. The BBB is composed of tight junctions between endothelial cells, which restrict the passage of ions and large molecules. The enzyme P-glycoprotein (P-gp) is also present in the BBB and actively transports certain drugs out of the brain, further limiting their entry.

**Why Each Wrong Option is Incorrect**
**Option A:** This drug is a lipophilic, small molecule that can easily cross the BBB. It is used to treat CNS disorders such as epilepsy and depression.
**Option B:** This drug is also lipophilic and can cross the BBB. It is used to treat conditions such as Parkinson's disease and restless legs syndrome.
**Option C:** This drug is a large, polar molecule that cannot cross the BBB. It is used to treat conditions such as hypertension and heart failure, but its effects are limited to the peripheral nervous system.

**Clinical Pearl / High-Yield Fact**
The ability of a drug to cross the BBB is a critical factor in its efficacy and safety. Drugs that cannot cross the BBB may not be effective in treating CNS disorders, but they can be used to treat peripheral nervous system disorders. It is essential to consider the BBB when selecting a drug for a patient with a CNS disorder.

**Correct Answer:** C. This drug is a large, polar molecule that cannot cross the BBB.