SBA — Single Best Answer · Review
Question 510
Endocrinology → General
Classification
Question
SSBAQuestion Header
Which physiological change helps explain why oxytocin is substantially more effective for induction of labour at term than in early pregnancy?
Question Stem
Select the single best answer.
Options
A
Progressive loss of placental oxytocinase
B
Increasing uterine sensitivity to oxytocin during pregnancy
C
Conversion of oxytocin into vasopressin near term
D
Suppression of maternal antidiuretic hormone near term
E
Increasing synthesis of oxytocin by the posterior pituitary
Explanation
Uterine sensitivity to oxytocin increases during pregnancy. Consequently, oxytocin can effectively induce labour at term, whereas the uterus is substantially less sensitive to its effects in early pregnancy.
Option Validity
A) Oxytocin is inactivated by placental oxytocinase, but the gestational difference in uterine response is attributable to increasing uterine sensitivity.
C) Oxytocin is not converted into vasopressin to produce its uterotonic effect.
D) Suppression of maternal antidiuretic hormone does not explain increasing uterine responsiveness to oxytocin.
E) Oxytocin is synthesized in the hypothalamus and transported for release from the posterior pituitary; increased posterior-pituitary synthesis is not the mechanism described.
Further Reading
Saxena, MRCOG-1 Basic Sciences, Endocrinology: Posterior Pituitary - Oxytocin.