Final answer:
The 1994 study involving the administration of AZT to pregnant HIV-positive women is an experiment because it involved the manipulation of a variable to observe its effects on HIV transmission to babies.
Step-by-step explanation:
In the 1994 study where 164 pregnant HIV-positive women were given AZT and another 160 were given a placebo resulting in different rates of HIV transmission to their babies, this scenario represents an experiment rather than an observation. This is because the study involved the active manipulation of a variable, which is the administration of AZT, to observe the effect on mother-to-child transmission of HIV. In an observational study, the researcher would only gather data without intervening. The results showed a marked decrease in the transmission rate among those who received AZT, highlighting its effectiveness.
Moreover, AZT's mechanism of action includes the inhibition of HIV's reverse transcriptase enzyme by mimicking the natural nucleoside, deoxythymidine triphosphate, but lacking a 3'OH group necessary for DNA strand elongation, thus leading to premature termination of viral DNA replication. In terms of mother-to-child transmission of HIV, the risk can be significantly reduced through the use of antiretroviral medications during pregnancy and after birth, along with cesarean delivery and avoiding breastfeeding if possible.