Final answer:
The BBB is a dynamic and highly selective structure composed of endothelial cells with tight junctions, which makes drug delivery to the CNS challenging due to restricted permeability of many substances, requiring specific transport mechanisms or molecular alterations.
Step-by-step explanation:
The most accurate statement regarding the blood-brain barrier (BBB) is that it is a highly selective and dynamic structure that regulates the entry of substances into the central nervous system (CNS). The BBB consists of endothelial cells joined by tight junctions, which, together with a thick basement membrane and astrocyte end feet, form a barrier that prevents most substances from passing freely into the brain and spinal cord.
While crucial nutrients such as glucose and amino acids are allowed passage, this barrier significantly impacts drug delivery because many drugs cannot permeate the BBB due to their size, polarity, or lack of specific transport mechanisms. Active transport is necessary for most molecules to cross the BBB, as simple diffusion is very limited.