## Core Concept
Facilitated diffusion is a type of passive transport that involves the movement of molecules across a cell membrane with the assistance of transport proteins, but without the use of energy. This process helps in the transport of large or charged molecules that cannot directly pass through the lipid bilayer.

## Why the Correct Answer is Right
Facilitated diffusion is characterized by the use of transport proteins (carriers or channels), movement down a concentration gradient, and it does not require energy. The correct answer, ., implies that facilitated diffusion does not involve active transport mechanisms or the use of energy.

## Why Each Wrong Option is Incorrect
- **Option A:** . This option is incorrect because facilitated diffusion indeed requires transport proteins to facilitate the movement of molecules across the membrane.
- **Option B:** . This option is incorrect as facilitated diffusion involves movement down a concentration gradient, not against it.
- **Option C:** . This option is incorrect because facilitated diffusion does not require energy, making it a form of passive transport.

## Clinical Pearl / High-Yield Fact
A key point to remember is that facilitated diffusion is crucial for the transport of glucose into cells, especially in the case of red blood cells, which use glucose transporter 1 (GLUT1) for this purpose. This process is vital for cellular metabolism and energy production.

## Correct Answer Line
**Correct Answer: D. .**