**Core Concept**
The brain's energy source during prolonged starvation is a critical adaptation to maintain its function. During starvation, the body initially relies on glucose as a primary energy source, but as glucose levels decrease, the brain adapts to utilize alternative energy sources. This adaptation is essential for the brain's survival during prolonged periods of starvation.

**Why the Correct Answer is Right**
The brain's energy source during later days of starvation shifts from glucose to ketone bodies, which are produced by the liver from fatty acid breakdown. This shift occurs due to the decrease in insulin levels and the increase in glucagon levels, leading to increased lipolysis and ketogenesis. The brain adapts to use ketone bodies as a primary energy source by increasing the expression of enzymes involved in ketone body metabolism, such as succinyl-CoA:3-ketoacid CoA transferase (SCOT) and acetyl-CoA acetyltransferase (ACAT). This adaptation allows the brain to maintain its function during prolonged periods of starvation.

**Why Each Wrong Option is Incorrect**
* **Option A:** Glucose is the primary energy source for the brain under normal conditions, but it is not the primary energy source during later days of starvation.
* **Option B:** Fatty acids can be used as an energy source by the brain, but they require conversion to acetyl-CoA, which is then used to produce energy through the citric acid cycle. However, the brain is not adapted to use fatty acids as a primary energy source during starvation.
* **Option D:** Amino acids can be used as an energy source by the brain, but they are not the primary energy source during later days of starvation.

**Clinical Pearl / High-Yield Fact**
It is essential to note that the brain's adaptation to use ketone bodies as an energy source during starvation can take several days to weeks to occur. This adaptation is critical for the brain's survival during prolonged periods of starvation.

**Correct Answer: C. Ketone bodies**