I think this mix-up comes from the fact that a lot of resources don't give the full story on ketone bodies. Some simply say that the three ketone bodies produced from FA breakdown are used as energy by some cells during starvation, and that's it. This is even the story I was told in BC in university.
But you guys are right, it's generally accepted that acetone is not used as a fuel source anywhere in the body and TBR must have an error in their text. Edit: I was wrong here. This seems to be a common misconception, even among textbook writers, but acetone can actually be used as a fuel source. Thanks to
@gettheleadout for the clarification and the source!
However, A is also an incorrect answer. The question is asking what the brain uses as a primary fuel
source. Acetoacetate and B-hydroxybutyrate are both taken up from the bloodstream by brain cells and then converted to Acetyl-CoA to be used for the CAC. Although Acetyl-CoA is involved, it is not travelling through the bloodstream, and is thus not taken up by brain cells as a fuel
source.
Though this may seem like a trivial distinction in wording, it is actually important. The MCAT expects you to understand
precisely what each question is asking, and not just what may be involved.
Finally, you mentioned that you thought Acetyl-CoA and acetoacetate are in equilibrium with each other. This is incorrect. Acetoacetate is converted to Acetyl-CoA by enzymes in (essentially) one direction.
