**Question:** Which of the following statements about a drug having high plasma protein binding is true?

A. A drug with high plasma protein binding has a lower unbound fraction in the circulation.
B. The drug's free fraction determines its ability to cross the blood-brain barrier.
C. A high protein binding drug does not contribute to the drug's pharmacological effect.
D. The unbound fraction of a drug is the part responsible for its side effects.

**Correct Answer:**

**Core Concept:** Plasma protein binding is the interaction between a drug and plasma proteins, primarily albumin and globulin. This binding influences the drug's pharmacokinetics and pharmacodynamics. High plasma protein binding can affect the drug's distribution, clearance, and therapeutic effects.

**Why the Correct Answer is Right:**

High plasma protein binding results in a lower unbound fraction of the drug circulating in the bloodstream. The unbound fraction is the part of the drug that can freely diffuse across biological membranes and interact with its target sites (receptors, enzymes, etc.). This is important because it determines the drug's access to its pharmacological targets and potential side effects. A low unbound fraction may indicate a drug with poor penetration into tissues or cellular compartments, while a high unbound fraction is associated with drugs that can effectively reach their targets and exert their therapeutic or side effects.

**Why Each Wrong Option is Incorrect:**

**A.** This statement is incorrect because a high protein binding drug has a higher unbound fraction, which allows it to interact with its targets and contribute to the drug's pharmacological effect.

**B.** The free fraction determines the drug's ability to cross membranes, not its contribution to the pharmacological effect. The drug's pharmacological effect depends on its interaction with specific receptors, enzymes, or other targets, not just its ability to cross the blood-brain barrier.

**C.** This statement is incorrect because binding to plasma proteins does not negate the drug's potential to cause side effects. The unbound fraction is crucial for side effects, as it is the part of the drug that is freely available to interact with unintended targets (e.g., off-target receptors, enzymes, etc.) and cause side effects.

**D.** This statement is incorrect because the unbound fraction is responsible for the drug's pharmacological effect and potential side effects. The unbound drug is the part that can interact with its targets and cause side effects.

**Core Concept:** The unbound fraction of a drug is essential for its pharmacological effect, side effects, and distribution to tissues and cellular compartments. The unbound drug is the part that can freely diffuse across biological membranes and interact with its target sites.