## Core Concept
The glycemic index (GI) is a measure used to rank foods on a scale from 0 to 100 based on how much they raise blood sugar levels after eating. It is a tool for assessing the quality of carbohydrates in food. The GI value is calculated by comparing the blood glucose response to a specific food with the blood glucose response to pure glucose.

## Why the Correct Answer is Right
The correct answer, **B. Glucose**, is right because the glycemic index is specifically calculated with respect to glucose. When calculating the GI of a food, the blood glucose response to that food is compared to the blood glucose response to pure glucose, which is given a GI value of 100. This allows for a standardized comparison of how quickly and how much different foods raise blood glucose levels.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because the GI is not calculated with respect to a generic or unspecified standard; it is specifically compared to glucose.
- **Option C:** This option is incorrect because, while starch can be a component of many foods and does affect blood glucose levels, the GI is specifically calculated in reference to glucose, not starch.
- **Option D:** This option is incorrect because fructose, although a sugar and a component of many foods, is not the standard used for calculating the GI; glucose is.

## Clinical Pearl / High-Yield Fact
A key point to remember is that foods with a low GI (less than 55) are considered to be those that cause a more gradual increase in blood glucose levels. Examples include whole grains, legumes, and most fruits and vegetables. This concept is crucial for managing blood sugar levels, particularly for individuals with diabetes.

**Correct Answer: B. Glucose**