Which of the following has the slowest rate of movement across the blood-brain barrier?  D-glucose   ethanol   glutamate   carbon dioxide   oxygen

The blood-brain barrier (BBB) is a highly selective semipermeable border of endothelial cells that prevents solutes in the circulating blood from non-selectively crossing into the extracellular fluid of the central nervous system where neurons reside.

The rate at which substances cross the BBB depends on their solubility in lipids (fat-solubility), their size, and whether or not they are transported by any of the various transport mechanisms that the BBB possesses.

1. D-glucose: Glucose is a primary source of energy for the brain. It crosses the BBB via facilitated diffusion through a specific glucose transporter protein, GLUT