Okay, let's tackle this question. The question is about the major fuel in the brain after several weeks of starvation. Hmm, I remember that the brain typically uses glucose, but when there's a lack of glucose, the body has to switch to other sources.

Wait, during starvation, the body starts breaking down glycogen first, but after a few days, glycogen stores are depleted. Then, the body starts breaking down fat into fatty acids. But wait, can the brain use fatty acids directly? I think not. Oh right, the brain can't use fatty acids as a fuel source because they can't cross the blood-brain barrier effectively. So what's the alternative?

Oh right, during prolonged starvation, the liver starts producing ketone bodies from fatty acids. Ketone bodies are water-soluble and can cross the blood-brain barrier. So after a few weeks, the brain switches to using ketone bodies as the primary fuel source. That makes sense because this spares muscle protein and allows the body to use fat stores instead.

Let me check the options. The correct answer is probably ketone bodies. The other options would be glucose, fatty acids, and maybe something else like lactate or amino acids. But glucose is not available after several weeks, and fatty acids can't cross the blood-brain barrier. So the correct answer is ketone bodies.

**Core Concept**
The brain's primary energy source shifts during prolonged starvation due to glucose scarcity. Ketone bodies (beta-hydroxybutyrate, acetoacetate) become the dominant fuel after 2–3 weeks when hepatic gluconeogenesis and ketogenesis are upregulated. This adaptation preserves muscle protein and utilizes fat stores efficiently.

**Why the Correct Answer is Right**
During starvation, glycogen stores deplete within 24–48 hours, forcing reliance on gluconeogenesis (from glycerol and amino acids) and ketone bodies. The brain initially uses glucose, but after 2–3 weeks, ketones produced by the liver from fatty acids (via beta-oxidation) become the major fuel. Ketones cross the blood-brain barrier via monocarboxylic acid transporters, providing ~60–70% of the brain’s energy needs in prolonged starvation.

**Why Each Wrong Option is Incorrect**
**Option A:** Glucose availability is insufficient after glycogen depletion.
**Option B:** Fatty acids cannot cross the blood-brain barrier in significant quantities.
**Option C:** Lactate is a minor fuel under normal conditions and not a starvation adaptation.

**Clinical Pearl / High-Yield Fact**
Remember **"Keto adaptation"** for NEET/USMLE: Ketone bodies (not fatty acids) are the brain’s primary fuel during prolonged fasting. This spares skeletal muscle protein, critical for survival. Use the mnemonic **"Ketones after 3 weeks"** to recall this timeline.

**Correct Answer: D. Ketone bodies**

✓ Correct Answer: C. b-Hydroxybutyrate