Final answer:
The substance that can disrupt the beat-to-beat variability of the fetal heart rate and delay temperature regulation in newborn infants is oxytocin (d). This hormone stimulates uterine contractions during labor, and excessive or prolonged action can lead to fetal distress.
Step-by-step explanation:
All oxytocin can disrupt the beat-to-beat variability of the fetal heart rate and delay the newborn infant's ability to regulate temperature. The correct answer is d) Oxytocin.
Oxytocin is a hormone that stimulates uterine contractions during labor. Throughout most of pregnancy, oxytocin hormone receptors are not expressed at high levels in the uterus. Toward the end of pregnancy, the uterus becomes more sensitive to oxytocin and the hormone is released through a positive feedback mechanism during childbirth. This process can lead to stronger and more frequent uterine contractions. If oxytocin's effect on the uterus is too powerful or prolonged, it can reduce the oxygenated blood flow to the fetus, causing fetal distress and affecting beat-to-beat variability in fetal heart rate.