Simple: glucose. (See its structure in this article.)
The reason for the this confusion is that glucose (C6H12O6) can be arranged in two different ways (technically, the two would be stereoisomers, should you care). However, only one of these two ways is biologically active.
And this biologically active form of glucose (D-glucose) is also known as... dextrose.
It really is all a bit of needless confusion, if you asked me.