The newborn of a lactating woman on fluoxetine developed CNS effects. What is the drug-induced alteration responsible for this?

The correct answer is associated with the immature blood-brain barrier in newborns, which is particularly significant in understanding how CNS effects can occur in infants exposed to substances through breastfeeding. The blood-brain barrier is a protective barrier that limits the passage of certain substances from the bloodstream into the brain, providing a critical defense mechanism for the central nervous system.

In neonates, this barrier is not fully developed, meaning that substances like fluoxetine, which can cross the blood-brain barrier more easily than in adults, can lead to central nervous system effects such as sedation, irritability, or other neurological symptoms. The immature nature of the barrier increases the risk of drug levels in the brain reaching concentrations that can lead to such effects, making this the primary factor in the newborn's adverse response to the drug.

Other factors, such as metabolite accumulation, increased maternal blood pressure, or altered drug absorption, do not specifically address the mechanism by which fluoxetine or its effects would manifest in the CNS of a newborn. These factors might influence therapeutic outcomes in different contexts but are less relevant to the particular vulnerability of a neonate's central nervous system when exposed to medications through breast milk.