Maternal urinary estriol excretion falls markedly when the fetus has anencephaly. The step of estriol synthesis that directly explains this finding is:

• A Conversion of cholesterol to pregnenolone in placental mitochondria
• B Placental conversion of progesterone to cortisol
• C Maternal hepatic aromatization of testosterone
• D Fetal adrenal secretion of dehydroepiandrosterone sulfate (DHEA-S) ✓

Explanation

The placenta lacks 17-alpha-hydroxylase and C17-20 lyase, so it cannot make androgens from progesterone. It therefore depends on fetal adrenal DHEA-S, hydroxylated at C16 by the fetal liver, as substrate for aromatization to estriol. Anencephalic fetuses have atrophic adrenal glands, so DHEA-S supply collapses and maternal estriol falls. Placental progesterone synthesis from maternal cholesterol is intact and irrelevant to this substrate dependency.

Reference: Williams Obstetrics, 26th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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