International Board of Certified Lactation Consultant (IBCLC) Practice Exam

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Which of the following substances has the lowest molecular weight?

Caffeine
Alcohol
Insulin
Albumin

The correct answer is: Caffeine

Caffeine is the correct choice as it has the lowest molecular weight among the substances listed. The molecular weight of caffeine is approximately 194.19 g/mol. In contrast, the other substances have significantly higher molecular weights: alcohol, specifically ethanol, has a molecular weight of about 46.07 g/mol, which is still higher than caffeine but lower than proteins; insulin has a larger molecular weight, around 5800 g/mol, and albumin has an even higher molecular weight of approximately 66,500 g/mol. This knowledge is essential for understanding pharmacokinetics and the behavior of different substances in the human body, especially in contexts such as lactation and maternal-infant health, where the molecular weight can influence absorption, transport, and metabolism. Caffeine's relatively small size allows it to cross biological membranes more easily, including the blood-brain barrier and potentially the placental barrier, which can have implications for both lactation and fetal development.