All of the following cross plasma membrane except:
**Core Concept**
The question is testing the ability to distinguish between substances that can cross the plasma membrane and those that cannot. This is a fundamental concept in pharmacology and physiology, as it affects the way cells interact with their environment and respond to stimuli.
**Why the Correct Answer is Right**
Substances that cannot cross the plasma membrane are typically large molecules, such as proteins or polysaccharides, that are too big to pass through the lipid bilayer. These molecules often require specialized transport mechanisms, such as endocytosis or active transport, to enter or leave the cell. In contrast, smaller molecules like ions, amino acids, and sugars can diffuse across the plasma membrane through passive transport mechanisms.
**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because, as a small molecule, it can diffuse across the plasma membrane through passive transport mechanisms.
**Option B:** Incorrect because, as a small molecule, it can diffuse across the plasma membrane through passive transport mechanisms.
**Option C:** Incorrect because, as a small molecule, it can diffuse across the plasma membrane through passive transport mechanisms.
**Clinical Pearl / High-Yield Fact**
Remember that the size and charge of a molecule determine whether it can cross the plasma membrane by diffusion. Large, charged molecules often require specialized transport mechanisms to enter or leave the cell.
**Correct Answer: D.**