This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Nutrition & Digestion, Subject name: Biochemistry
After a fast of a few days, ketone bodies become an important fuel 
Select and explain. 
A. Liver, B. Brain, C. Skeletal muscle, D. Red blood cells
B: Brain
Answer: B: Brain 
Explanation: The brain begins to use ketone bodies when levels start to rise after 3 to 5 days of fasting. Normally, the brain will use only glucose as a fuel (most fatty acids cannot cross the blood-brain barrier to be metabolized by the brain), but when ketone bodies are elevated in the blood, they can enter the brain and be used for energy.