**Core Concept**
Epimers are carbohydrates that differ in configuration at only one carbon atom. In the context of glucose, epimers can arise from changes in the configuration at any of the six carbon atoms, but the most relevant epimer of glucose is mannose, which differs at the carbon-2 position.

**Why the Correct Answer is Right**
Mannose is an epimer of glucose because it has the same molecular formula (C6H12O6) and the same overall structure, but with a difference in the configuration at the C-2 carbon atom. This difference in configuration results in a distinct set of chemical and physical properties for mannose compared to glucose. The epimerization at the C-2 carbon atom is significant because it affects the reactivity and metabolic fate of the sugar.

**Why Each Wrong Option is Incorrect**
**Option B:** Glyceraldehyde is not an epimer of glucose because it has a different molecular formula (C3H6O3) and a distinct overall structure, with only three carbon atoms instead of six. Glyceraldehyde is a simple three-carbon sugar that is not a direct epimer of glucose.
**Option C:** Fructose is not an epimer of glucose because it has a different molecular formula (C6H12O6) and a distinct overall structure, with a five-membered ring instead of the six-membered ring of glucose. Fructose is a ketohexose that is not a direct epimer of glucose.
**Option D:** The statement "None" is incorrect because mannose is indeed an epimer of glucose, as explained above.

**Clinical Pearl / High-Yield Fact**
When examining carbohydrate metabolism, it is essential to remember that epimerization at specific carbon atoms can significantly affect the reactivity and metabolic fate of sugars. This knowledge is crucial for understanding various biochemical pathways and reactions.