**Core Concept**
Glucose is the primary energy source for tissues, especially the brain, during the postoperative period. After surgery, the body undergoes a stress response, increasing glucose demand and depleting glucose stores. This is due to the release of stress hormones such as cortisol and adrenaline, which elevate blood glucose levels.

**Why the Correct Answer is Right**
The liver stores glycogen, which is broken down into glucose during the postoperative period. However, the liver's glycogen stores are depleted rapidly, especially in the first 24-48 hours after surgery. This is because the liver's glycogenolysis pathway is activated, releasing glucose into the bloodstream. The brain, being a glucose-dependent organ, relies heavily on this glucose supply. As glycogen stores are depleted, the body begins to break down fat and protein for energy, leading to a shift in metabolism.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the liver's glycogen stores are depleted first, not the fat stores. While fat stores do play a role in energy supply, they are not the primary source of energy in the postoperative period.
**Option B:** This option is incorrect because the body's protein stores are not depleted first in the postoperative period. While protein catabolism does occur, it is a secondary process that occurs after glycogen stores are depleted.
**Option C:** This option is incorrect because the kidneys' function is not directly related to glucose depletion in the postoperative period. While the kidneys do play a role in glucose regulation, they are not the primary source of glucose in the body.

**Clinical Pearl / High-Yield Fact**
In the postoperative period, it is essential to maintain glucose levels within a normal range to prevent glucose depletion and subsequent catabolism of fat and protein stores.

**Correct Answer:** A. Glycogen