**Core Concept**
In burn patients, various physiological and biochemical changes occur due to the release of inflammatory mediators, cytokines, and hormones. These changes can affect multiple organ systems, leading to a complex clinical picture. The burn response is characterized by a systemic inflammatory response syndrome (SIRS) and a hypermetabolic response.

**Why the Correct Answer is Right**
The correct answer is related to the concept of decreased levels of certain substances in burn patients. In burn injuries, the release of inflammatory mediators and the activation of the hypothalamic-pituitary-adrenal (HPA) axis lead to an increase in cortisol levels. Cortisol has a negative feedback effect on the HPA axis, which results in an increase in cortisol levels. This increase in cortisol levels is a key component of the hypermetabolic response seen in burn patients.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the decrease in certain substances in burn patients is not related to this option.
**Option B:** This option is incorrect because the decrease in certain substances in burn patients is not related to this option.
**Option C:** This option is incorrect because the decrease in certain substances in burn patients is not related to this option.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that burn patients often experience a hypermetabolic response, characterized by an increase in cortisol levels, insulin resistance, and a catabolic state. This response can lead to muscle breakdown, weight loss, and decreased wound healing.

**Correct Answer:** D.