The following substances pass into the blood of the foetus from the mother's blood via the placenta except

Analyze the question: We need the substance that does NOT pass from the mother's blood to the foetus's blood across the placenta.

Concept: The placenta is the organ where the mother's blood and the foetal blood come close together (without mixing) so that exchange can happen by diffusion, osmosis and active transport.

• Mother → foetus: oxygen, glucose, amino acids, vitamins, mineral salts, water, antibodies (passive immunity), and some harmful substances such as alcohol, drugs, nicotine, and certain viruses (e.g. HIV, rubella virus, Zika).
• Foetus → mother: carbon dioxide and urea (nitrogenous waste), which the mother's blood carries away to be excreted.

Going through the options:

• A. Carbon dioxide: This is a waste product of the foetus's respiration. It passes from the foetal blood to the mother's blood, not the other way round. So it is the exception.
• B. Glucose: Passes from mother to foetus as an energy source. ✔
• C. Viruses: Some viruses (e.g. rubella, HIV) can cross the placenta and infect the foetus. ✔
• D. Antibodies: They pass from mother to foetus and give the baby temporary immunity. ✔

Answer: The question asks for the substance that does not pass into the foetal blood from the mother, and that is carbon dioxide.

Exam tip: Remember the direction of flow: "Food and oxygen go in, waste (CO₂ and urea) goes out." When you see "except" in a question, find the one option that goes the opposite way or does not belong.