## Core Concept
In starvation, the body's primary goal is to maintain energy supply to vital organs. During fasting or starvation, the body sequentially utilizes different substrates for energy production, starting with glucose, then glycogen, and finally fat.

## Why the Correct Answer is Right
The correct answer, **Muscle**, is the last to disappear or be significantly affected in terms of protein breakdown during starvation. This is because muscle protein is relatively spared until late in starvation. Initially, the body uses stored glucose and glycogen. Once these are depleted, it begins to break down fat for energy. Only when these sources are insufficient does the body start to break down muscle protein for energy through gluconeogenesis. This process helps maintain blood glucose levels for the brain and other tissues that prefer or exclusively use glucose for energy.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because the brain's utilization of glucose changes early in starvation; it adapts to use ketone bodies for energy, reducing its dependency on glucose.
- **Option B:** This option is incorrect because liver glycogen is one of the first sources of energy to be depleted, typically within 12-24 hours of fasting.
- **Option C:** This option is incorrect because, although fat stores are mobilized early in starvation for energy, the question seems to focus on the last tissue or component to be affected or utilized.

## Clinical Pearl / High-Yield Fact
A key point to remember is that during starvation, after depletion of glycogen stores (which typically lasts about 24 hours), the body enters a phase where it primarily uses fatty acids and ketone bodies for energy. Muscle protein is preserved as long as possible, being used only when there's a significant need for amino acids for gluconeogenesis. This adaptation helps in understanding why certain conditions may develop during prolonged fasting or starvation.

## Correct Answer: D. Muscle