MCAT Biology Question 35: Answer and Explanation

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Question: 35

5. Which of the following is likely to be found in maternal blood during pregnancy?

  • A. Immunoglobulins produced by the fetus
  • B. Fetal hemoglobin released from fetal red blood cells
  • C. Progesterone produced by placental cells
  • D. Carbon dioxide exhaled from fetal lungs

Correct Answer: C

Explanation:

During pregnancy, the placenta produces estrogen and progesterone to maintain the endometrium. These hormones are necessary for proper gestation of the fetus and should be measurable in maternal blood because they act on maternal organs. Prior to birth, the fetus is immunologically na?ve and does not yet produce immunoglobulins, eliminating choice (A). It is worth noting, though, that maternal immunoglobulins cross the placenta to enter fetal blood. Fetal hemoglobin is a large protein and, thus, cannot easily cross the placenta. Further, red blood cells are much too large to cross the barrier themselves, eliminating choice (B). Carbon dioxide from fetal metabolism can be found in maternal blood, but the lungs are nonfunctional prior to birth as the fetus is suspended in amniotic fluid. Carbon dioxide is transferred across the placenta directly from the fetal bloodstream, eliminating choice (D).

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