Maternal serum estriol rises steeply in the third trimester. The immediate precursor that the fetus MUST supply for this rise is:

• A Cholesterol synthesized in the fetal liver
• B Testosterone from fetal Leydig cells
• C Pregnenolone secreted by the fetal corpus luteum
• D Dehydroepiandrosterone sulfate from the fetal adrenal cortex ✓

Explanation

The placenta lacks 17-alpha-hydroxylase and C17-20 lyase, so it cannot convert progesterone to androgens. It therefore depends on fetal adrenal dehydroepiandrosterone sulfate, which is 16-alpha-hydroxylated in the fetal liver and then aromatized by placental sulfatase and aromatase to estriol. This feto-placental unit explains why estriol falls sharply after fetal demise. Cholesterol supports progesterone synthesis, not estriol.

Reference: Williams Obstetrics, 26th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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