**Core Concept:** Energy expenditure in the resting state is determined by the amount of energy required to maintain body functions and homeostasis. This includes maintaining body temperature, circulating blood, and various enzymatic reactions.

**Why the Correct Answer is Right:** Energy expenditure in the resting state is primarily influenced by a person's body mass index (BMI). A higher BMI results in increased energy expenditure due to the increased body mass and the need for more energy to maintain bodily functions. The correct answer, **D**, represents the relationship between resting energy expenditure (REE) and body mass index (BMI).

**Why Each Wrong Option is Incorrect:**

A. Resting energy expenditure is not directly proportional to muscle mass. Although muscle mass contributes to overall body mass, the relationship between BMI and REE is more significant than muscle mass alone.

B. This option is incorrect as it states that resting energy expenditure is solely determined by heart rate. Although heart rate contributes to REE, it is not the sole factor.

C. This option is incorrect as it suggests that resting energy expenditure is solely determined by heart rate. Heart rate is a component of REE, but other factors, such as BMI, also play a significant role.

**Clinical Pearl**: Understanding the relationship between BMI and resting energy expenditure is crucial for clinical practice, as it helps in determining an individual's daily caloric requirement, which is essential in diet planning and weight management.

**Correct Answer:** **D.** Resting energy expenditure (REE) is directly proportional to the square of body mass index (BMI).

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In summary, the correct answer (D) highlights the relationship between resting energy expenditure (REE) and body mass index (BMI). While muscle mass contributes to body mass, the stronger relationship exists between REE and BMI, indicating that a higher BMI results in increased energy expenditure. This relationship is essential in determining an individual's daily caloric requirements for diet planning and weight management.