## **Core Concept**
The energy expenditure during normal breathing is relatively low and is primarily related to the work of moving air in and out of the lungs. This process involves the contraction of the diaphragm and other accessory muscles, which consumes a minimal amount of energy under normal conditions.

## **Why the Correct Answer is Right**
The correct answer, , indicates that the energy expenditure during normal breathing is approximately 2-3% of the total body energy expenditure. This low percentage reflects the efficient design of the respiratory system, which allows for the exchange of gases with minimal effort under resting conditions. The diaphragm, the primary muscle used for breathing, contracts and relaxes with a very low energy cost.

## **Why Each Wrong Option is Incorrect**
- **Option A:** - This option suggests a much higher energy expenditure than what is observed during normal breathing. Such high energy expenditure would be more characteristic of intense physical activity rather than quiet breathing.
- **Option B:** - This option implies a moderate energy expenditure that might be seen during light exercise or specific conditions that increase the work of breathing, not during normal, relaxed breathing.
- **Option D:** - This option implies an even lower energy expenditure than , which, while closer, still does not accurately represent the commonly accepted value for the energy cost of quiet breathing.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the work of breathing (WOB) increases significantly under conditions that affect lung compliance or airway resistance, such as chronic obstructive pulmonary disease (COPD) or pneumonia. In these conditions, the energy expenditure for breathing can increase substantially, making breathing more laborious.

## **Correct Answer:** .