NBME 5 block2 question, please explain, thanks. - nicolecat

[ArchivalUser]

an 11 year old boy has been passing large volumes of urine and has been extremely thirsty over the past 10 days. he has lost a considerate amount of weight despite having a good appetite. during the past 2 days, he has become increasingly drowsy, lethargic, and difficult to arouse. respirations are deep and rapid. his blood pressure is 100/60 mm Hg while supine and 40 mm Hg/undetectable while sitting. In addition to glucose, which of the following is the principal fuel oxidized in his brain?
a. alanine
b.glutamate
c.glycogen
d.B-hydroxybutyric acid
e.palmitate
f.triacylglycerol

I know the answer is D, but why "his blood pressure is 100/60 mm Hg while supine and 40 mm Hg/undetectable while sitting"? Thanks

[ArchivalUser]

because he is hypovolemic.