After prolonged starvation lasting several weeks, what becomes the primary energy source for the brain?

Explanation

During extended periods of starvation, the brain shifts from using glucose to ketone bodies for energy. Beta-hydroxybutyrate, a type of ketone body, becomes the main fuel source, as glucose availability is limited. Fatty acids cannot cross the blood-brain barrier efficiently, and glycerol and amino acids are not primary fuels for the brain in this state.