Okay, let's tackle this question. The question is about what the body utilizes during starvation. I need to remember the metabolic changes that happen when the body is deprived of nutrients.

First, the core concept here is the body's metabolic adaptation to starvation. Initially, the body uses glucose from glycogen stores. But once those are depleted, it switches to other sources. I think the main shift is to using fat stores, breaking them down into fatty acids which are then converted into ketone bodies. The liver produces ketones, which the brain can use as an alternative energy source when glucose is scarce.

The correct answer should be ketone bodies. Let me check the options. Wait, the options aren't provided, but the correct answer is mentioned as an example. Assuming the correct answer is D. Ketone bodies.

Why is ketone bodies correct? During prolonged starvation, glycogen stores are exhausted after a day or two. The body starts breaking down fat into fatty acids, which are then converted into ketone bodies in the liver via ketogenesis. These ketones can cross the blood-brain barrier and provide energy to the brain, reducing the need for glucose. This is crucial because the brain can't use fatty acids directly.

Now, the wrong options. If the options included glucose, that's incorrect because after a few days, glucose from glycogen is gone. If there's an option like muscle protein, that's used later in prolonged starvation when ketones are no longer sufficient, but that's a later stage. Another possible wrong option is fatty acids, but the brain can't use them, so the body makes ketones instead. Also, if there's an option like glycogen, that's only for the initial phase.

Clinical pearl: Remember that in early starvation, the body uses glycogen, then shifts to fat and ketones, and in prolonged cases, protein breakdown occurs. This is important for understanding conditions like diabetic ketoacidosis or managing patients with prolonged fasting.

So the correct answer is D. Ketone bodies.

**Core Concept**
During prolonged starvation, the body transitions from glucose-dependent metabolism to lipid and ketone-based energy systems. The liver converts fatty acids into **ketone bodies (acetoacetate, β-hydroxybutyrate, acetone)** to supply energy to the brain and other tissues when glucose availability is limited.

**Why the Correct Answer is Right**
After glycogen stores are depleted (typically within 24–48 hours), the body initiates **lipolysis** of adipose tissue, releasing free fatty acids. The liver metabolizes these into **ketone bodies via ketogenesis**, which cross the blood-brain barrier to replace glucose as the brain’s primary fuel. This spares muscle protein catabolism and sustains energy demands during prolonged caloric restriction.

**Why Each Wrong Option is Incorrect**
**Option A:** *Glucose* is incorrect because endogenous glucose production (from glycogen) ceases after 1–2 days of starvation.
**Option B:** *Muscle protein* is a last-resort energy source in extreme starvation, not the primary fuel during early to moderate fasting.
**Option C:** *Fatty acids* cannot cross the blood-brain barrier; thus, they cannot directly supply energy to the brain.

**Clinical Pearl / High-Yield Fact**
Ketone bodies are the body’s "emergency fuel

✓ Correct Answer: A. Ketone bodies