Which statement best describes distribution differences between neonates and adults?

• A. Neonates have lower total body water than adults
• B. Adults have higher total body water than neonates
• C. Elderly have higher tissue perfusion than neonates
• D. Neonates have higher total body water, lower fat content, and lower plasma protein binding

Answer: (D)

Distribution is shaped by body composition and how much of a drug is bound to plasma proteins. Neonates have a higher proportion of total body water than adults, which expands the volume of distribution for hydrophilic drugs. They also have lower fat content, reducing the distribution volume for lipophilic drugs. Additionally, plasma protein binding is reduced in neonates due to lower levels and differing binding affinities, so a larger fraction of drug remains unbound and available to distribute and be cleared. Taken together, these factors mean neonates generally exhibit higher total body water, lower fat, and lower protein binding compared with adults, making this description the best fit of the statements. The other options misstate the neonatal body composition or compare groups in a way that isn’t supported by typical physiology.