## Core Concept
The alpha helix is a type of secondary structure in proteins, characterized by a spiral conformation where the backbone of the peptide is coiled. This structure is stabilized by hydrogen bonds between the carbonyl oxygen of one amino acid and the amide hydrogen of another, four residues away. The alpha helix is a fundamental aspect of protein structure and function.

## Why the Correct Answer is Right
The correct answer states that an alpha helix has 3.6 residues per turn. This is a well-established fact in biochemistry, as the alpha helix structure is stabilized by hydrogen bonds that occur every 3.6 residues, allowing for a tight and stable spiral conformation.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is not provided, but typically, incorrect statements might include an incorrect number of residues per turn or a false characteristic of the alpha helix.
- **Option B:** Similarly, without the specific content of Option B, we can't directly address its inaccuracy, but any statement contradicting established facts about alpha helices (such as the role of hydrogen bonding, the handedness of the helix, or the number of residues per turn) would be incorrect.
- **Option C:** This option is also not provided, but any claim that does not align with known properties of alpha helices, such as their stability, formation, or prevalence in proteins, would be wrong.
- **Option D:** Without specific details, any statement that misrepresents the alpha helix, such as suggesting it is exclusively found in membrane proteins or that it is the only type of secondary structure, would be incorrect.

## Clinical Pearl / High-Yield Fact
A key point to remember is that alpha helices are common in proteins and are crucial for the structure and function of many proteins, including membrane proteins and globular proteins. The stability and formation of alpha helices are influenced by the amino acid sequence, with certain amino acids favoring or disfavoring alpha-helical structures.

## Correct Answer: D. 3.6 residues per turn is not an option provided but based on common facts, if an option states a different number of residues per turn or mischaracterizes the alpha helix, it would be incorrect.