Through which barriers can lipid-soluble drugs typically pass?

Lipid-soluble drugs are characterized by their ability to easily pass through biological membranes due to their chemical properties. The correct choice highlights that cell membranes, which are primarily composed of a lipid bilayer, allow for the diffusion of lipid-soluble substances directly across them.

This process is facilitated by the hydrophobic nature of lipid-soluble drugs, which enables them to interact favorably with the lipid components of cell membranes. Unlike water-soluble drugs, which may require specific transport mechanisms, lipid-soluble drugs can traverse membranes freely without needing a carrier protein or specific channels.

In contrast, while lipid-soluble drugs can also impact the blood-brain barrier, gastrointestinal tract, and liver tissues, these routes involve additional complexities such as transport mechanisms or permeability considerations unique to those specific barriers. The essence of the question lies in identifying the primary biological interface that lipid-soluble drugs can pass through with relative ease, making the passage through cell membranes the most straightforward and direct mechanism for these drugs.