Final answer:
The blood-brain barrier (BBB) is the structure that prevents the passage of certain substances, including some drugs and pathogens, to the CNS, allowing only essential nutrients to pass through.
Step-by-step explanation:
The structure that slows or prevents the passage of some drugs and disease-causing organisms to the Central Nervous System (CNS) is the blood-brain barrier (BBB). This physiological barrier restricts what can cross from circulating blood into the CNS, which includes a highly selective membrane formed by endothelial cells. The blood-brain barrier allows essential nutrients like glucose and amino acids to pass through, while most other substances, including many pharmaceuticals and white blood cells, cannot easily cross this barrier, protecting the brain from potential harm.