**Core Concept**
The brain's energy source during prolonged starvation involves a shift from glucose to alternative substrates due to decreased glucose availability. This adaptation is crucial for the brain's survival during periods of caloric restriction.

**Why the Correct Answer is Right**
During the later part of starvation, the brain's primary energy source transitions from glucose to ketone bodies, which are produced by the liver from fatty acids. This shift is mediated by the low glucose levels and the high levels of ketone bodies in the bloodstream. The brain's ability to use ketone bodies as an energy source is facilitated by the enzyme succinyl-CoA:3-ketoacid CoA transferase (SCOT), also known as thiophorase, which is involved in the conversion of acetoacetate to acetoacetyl-CoA.

**Why Each Wrong Option is Incorrect**
**Option A:** Glucose is the primary energy source for the brain under normal conditions, but not during prolonged starvation.
**Option B:** Fatty acids cannot directly cross the blood-brain barrier, making them an unlikely energy source for the brain.
**Option C:** Lactate is produced during glycolysis, but it is not a primary energy source for the brain during starvation.

**Clinical Pearl / High-Yield Fact**
The brain's ability to use ketone bodies as an energy source during starvation is a critical adaptation that allows it to function for extended periods without glucose. This phenomenon is often referred to as "ketosis" and is a hallmark of the brain's response to caloric restriction.

**Correct Answer: C. Lactate**