**Core Concept**
The transport of drugs through biological membranes involves various mechanisms to facilitate the movement of molecules across cell membranes, which is crucial for drug efficacy and distribution. Biological membranes are semi-permeable barriers composed of a phospholipid bilayer, proteins, and cholesterol, allowing for selective permeability and regulating the movement of substances.

**Why the Correct Answer is Right**
The correct answer, "All of the above," accurately reflects the multiple mechanisms by which drugs traverse biological membranes. **Passive diffusion** (Option A) occurs through the movement of molecules down their concentration gradient without the need for energy, facilitated by the lipid solubility of the drug. **Facilitated diffusion** (Option B) involves carrier proteins and channel proteins that assist in the transport of molecules across the membrane, often against their concentration gradient. **Active transport** (Option C) requires energy, usually in the form of ATP, to move molecules against their concentration gradient, often using carrier proteins or pumps.

**Why Each Wrong Option is Incorrect**
**Option A:** Passive diffusion is a specific type of transport mechanism, whereas the question asks for a comprehensive answer that includes all possible methods of transport.

**Option B:** Facilitated diffusion is another specific mechanism, and while it is indeed a method of transport, the correct answer encompasses all types of transport, not just facilitated diffusion.

**Why Each Wrong Option is Incorrect (continued)**
**Option C:** Active transport is also a specific mechanism that requires energy, whereas the correct answer includes all types of transport, including active transport.

**Clinical Pearl / High-Yield Fact**
When considering the transport of drugs across biological membranes, it's essential to remember that lipid solubility (for passive diffusion) and the presence of carrier proteins or pumps (for facilitated diffusion and active transport) significantly influence the efficacy and distribution of drugs.

**Correct Answer Line**
✓ Correct Answer: D. All of the above