Tobacco smoke components impairing fetal oxygen supply

Steps:

• Smoking reduces fetal oxygen by affecting maternal blood and placental flow.
• Carbon monoxide binds to hemoglobin, forming carboxyhemoglobin that carries less oxygen.
• Nicotine constricts blood vessels, limiting placental blood flow and oxygen delivery.
• Tar deposits in lungs but does not directly impact oxygen transport or flow.

Why A is correct:

• Carbon monoxide reduces oxygen-carrying capacity of blood (binds hemoglobin 200 times more tightly than oxygen); nicotine causes vasoconstriction, decreasing placental perfusion.

Why the others are wrong:

• B: Tar clogs lungs long-term but does not reduce oxygen supply acutely.
• C: Tar lacks direct role in oxygen reduction, unlike carbon monoxide alone.
• D: Tar irrelevant to oxygen; nicotine alone insufficient without carbon monoxide's binding effect.

Final answer: A