If starvation exceeds 7 days, the major nutritional supply of the brain comes from
**Core Concept**
When a person starves for an extended period, the body's primary energy sources (glucose and glycogen) are depleted, and the brain, which is highly dependent on glucose for energy, must adapt to use alternative energy sources.
**Why the Correct Answer is Right**
Beyond 7 days of starvation, the brain begins to utilize ketone bodies, which are produced in the liver from fatty acid breakdown, as its primary energy source. This adaptation occurs due to the activation of enzymes involved in beta-oxidation and the inhibition of enzymes involved in glycolysis. As a result, the brain's energy demand is met by the increased production and utilization of ketone bodies.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because, although the liver does produce ketone bodies, it is not the primary site of ketone body utilization in the brain during prolonged starvation.
* **Option B:** This option is incorrect because the brain's primary energy source during starvation is not amino acids, which are broken down from muscle protein.
* **Option C:** This option is incorrect because the brain does not use lactate as its primary energy source during prolonged starvation.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that the brain's adaptation to use ketone bodies as its primary energy source during prolonged starvation can lead to a condition known as ketoacidosis, which can be life-threatening if not properly managed.
**Correct Answer: C. Ketone bodies.**