An infant who receives a drug that does not produce CNS side effects in adults exhibits drowsiness and sedation. The nurse understands that this is because of differences in which physiologic system in infants and adults?

A. Gastrointestinal absorption differences can affect how much of a drug enters the bloodstream, but they do not fully explain why a drug causes CNS effects in infants but not in adults. Absorption mainly affects drug concentration, not sensitivity of the central nervous system. B. The blood-brain barrier (BBB) in infants is immature and more permeable than in adults. This allows drugs that normally do not cross into the CNS in adults to enter the brain in infants, leading to drowsiness, sedation, or other CNS effects. The underdeveloped BBB is a key factor in neonatal and infant drug sensitivity, especially for medications that are otherwise CNS-inactive in adults. C. The first-pass effect refers to drug metabolism in the liver before it reaches systemic circulation. While this affects drug bioavailability, it does not explain why a drug specifically causes CNS effects in infants. D. Renal filtration affects drug elimination, but slower renal clearance mainly increases systemic drug exposure, not selective CNS penetration. It is not the primary reason for unexpected sedation in infants.